This delicious and versatile thing came about a couple of months ago in a confluence of leftovers and email. The leftovers were of a creamy potato gratin. The email exchange related to New York restaurant recommendations, specifically to the irresistible Alsatian-style tarte flambée served in the bar room of The Modern and devised by Gabriel Kreuther, who was chef of the restaurant until his recent departure to open his own place (to which I look forward). Mr. Kreuther's tart exemplifies this dish in its most elegant but still authentic form: a thin, crisp slightly leavened disc garnished with crème fraîche, fromage blanc, bacon and onions (the recipe has been published). It is the sort of dish that stays in the mind once you've eaten it, with its slightly crunchy onions and smoky bacon and the creamy bed on which they lie.
It was those elements that came to mind as I regarded a plate of leftover potato gratin one morning. When warmed through, it would be creamy, and if potatoes, onions and bacon aren't good friends, then friendship has lost its meaning. Plus, potatoes are a first-rate pizza or focaccia topping (or savory pie/turnover filling). So I made pizza dough (no time at all in the food processor), then at dinner time stretched it thin, spread it with potato gratin and topped this with bacon matchsticks and thinly sliced onions, both partially cooked. It was very good, and I made two or three similar things until I'd worked out a procedure that didn't involve leftover potato gratin (which Jackie and I do not always have). Here's how I made it the other day, in its most recent version.
In the food processor I made pizza dough using 1-1/2 cups (maybe 190 g) flour, plenty of salt, around 3/4 teaspoon instant yeast and enough room-temperature water to form a soft, almost wet dough, into which I let the machine knead around 2 tablespoons good olive oil. I put it into a lidded container to rise. This would eventually yield a roughly 10 x 14 inch (25 x 35 cm) tart, enough for dinner for three sensible people. The other elements too were made in advance. I peeled, halved and thinly sliced two medium onions and, having found them a trifle harsh as I cut them, blanched them in boiling water for just a few seconds (a useful trick if you want to soften the flavor) before putting them in a sieve, rinsing in cold water and draining. I cut 2 ounces (60 g) of smoked bacon into thin matchsticks -- you could use more if you wanted to, but not more than 3 ounces, I'd say -- and rendered out some of its fat over medium-low heat, pouring off the fat but saving it just in case. To this I added the onions (and salt and pepper) and stirred them over medium heat for a couple of minutes: they should be only partly cooked at this stage. I found them a trifle austere fat-wise, so returned a little of the bacon grease to the pan. I left the mixture to cool.
I peeled 1-1/4 pounds (say, 575 g) of Carola potatoes (you could use something like German butterballs or Yukon golds with equal success, though russets would probably not work as well), halved them and cut them into 1/4-inch (generous 6 mm) slices, put them into a pan with one minced clove of garlic, salt, pepper, nutmeg, rosemary (not my first choice: use thyme if you've got it, please) and a mixture of milk and cream to cover. This I simmered for about 12 minutes, until the potatoes were tender. Using a slotted spoon, I fished out the potatoes and put them into a bowl to cool while I slowly reduced the milk-cream mixture until it had thickened to a nice creamy consistency. To this I added around 2 ounces (60 g) gruyère cheese, grated, and stirred until it melted; I poured the sauce over the potatoes and left them aside until dinner time.
An hour and a quarter before dinner, I set the oven's dial to 500 degrees F (260 C) and let it heat for an hour, with a large pizza stone installed (the stone is optional but recommended). Toward the end of this time I stretched the dough to its approximately 10 x 14 inch size and laid it onto a lightly floured pizza peel to rest and rise a bit for five or ten minutes. Note that you can bake this in a sheet pan too, which avoids the risk of oozing sauce burning in the oven. When the dough had rested and become tractable once again, I tweaked its size and shape, then topped it with the potatoes and enough of the sauce to assert itself -- most of it, as it happened. Atop this I distributed most of the onion-bacon mixture (the leftovers are in a meatloaf mixture as I write); for spreading both ingredients, the best, perhaps the only, tool is fingers.
I baked it for 14 minutes, but so much will depend on your oven and on whether you are baking it in a pan or right on a pizza stone: Just make sure the crust is thoroughly cooked, the bottom crisp and the edges of some of the onions, potatoes and bacon caramelized. I let it cool a few minutes before cutting it into rectangles (they could have been wedges or any other shape).
It made a delightful dinner, with qualities drawn from pizza, from tarte flambée and, yes, from potato gratin. You could serve a salad with it, I suppose. At the outset I called this tart "versatile," and it is: it's a standalone meal; it's a first course (what a first course!); and, believe it or not, it's a side dish in place of potatoes, so long as you remove thicker outer crusts and serve it in modest portions. In fact, cut it into little squares and it becomes a drinking snack.
It's amazing how inspiring leftover spuds can be, don't you think?