Sift the flour and make a well in the center. Add the salt, water, egg yolk and cubed butter. Mix these together with your fingertips in the center of the well. Gradually bring the ingredients together. Knead very briefly if needed. Roll the pastry into a ball without working the dough any further. Cover with plastic wrap. Refrigerate so that the dough becomes firmer and loses any remaining elasticity.
Filling
Trim off the rind and any cartilage from the smoked pork belly. Cut the slices across the grain into inch-long strips (or into small lardons). Quickly sauté the pork strips in the oil over high heat until just cooked. Be sure not to dry them out. Drain on paper towels.
Custard
Mix the eggs, egg yolks, cream, and milk together in a small bowl. Season with salt, cayenne pepper, and a few scrapings of nutmeg.
Crust
Butter and flour a tart mold and place it in the fridge to chill. Roll the dough out to a thickness of 1/16th of an inch. Dock it all over. Line the tart pan with the dough. Brush away any excess flour. Crimp the edges of the dough.
Baking
Scatter the bacon and the Gruyère evenly into the crusts. Pour the custard mixture over them. Bake in a 350°F convection oven for 30 minutes. Serve cold or warm.