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1
In a food processor, combine the flour and salt. Cut the butter into roughly tablespoon-sized pieces and add, then pulse until the mixture looks a bit crumbly. Put in the egg yolk and about 3 tablespoons of the water, then run the processor until the dough starts to come together. It should clump into a big ball; if it doesn't, drizzle in another tablespoon of the water. You want to add as little liquid as possible.
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2
Remove from the food processor and flatten into two 7-inch disks. Wrap each in plastic wrap and refrigerate about 5 minutes.
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3
In the meantime, make the filling: Saute the onions in the oil over a medium flame, stirring very infrequently. Once they soften and begin to brown on each side, return the flame to low and allow to slowly caramelize. During this time, it is important to make sure the onions are spread as evenly as possible across the pan. Every few minutes, scrape the bottom and redistribute the onions so each gains the maximum amount of surface area. The intention is to slowly crisp the onions by enticing the remaining liquids to sweat out, and for the onions to sweeten by condensing in their own juices. If you stir too often, the onions will turn to mush. This process takes about 40 minutes.
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4
When the onions are dark brown, but not burnt, add the thyme, cumin, and season with salt and a touch of cayenne. Cook for another 2 minutes, then set aside.
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5
When ready to make the tarts, preheat the oven to 400u00b0F. Roll out each disk into a rough circle. No need to be perfect, you just want the dough fairly even. You can either lift each rolled-out dough into a tart pan with removable bottom, or you can simply fold about 1 inch of the edges in all the way around, pinching every inch or so to seal. If the latter, slide onto a parchment-lined baking sheet.
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6
Mix together the creme fraiche and the egg yolk and season with freshly ground pepper. Spread it evenly across the two tarts.
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7
Spread each tart with about 3/4 cup of the onion filling. Sprinkle each with 1 tablespoon of the diced butter. Bake the tarts 20-30 minutes, until the crust is golden. Garnish with fresh thyme, then cut each into 6 wedges and serve.
