Combine the flour, salt and yeast in a large bowl. Add the oil and warm milk and mix until a rough dough is formed. Dump dough out onto a work surface dusted with a little extra flour and knead until the dough is smooth and silky. Add a small amount of oil to a clean bowl, and transfer the dough to the bowl. Cover and allow to rise until doubled in size, about an 1½ hours.
While dough is rising, add chopped bacon to a cold skillet. Cook over medium heat until the bacon is rendered and just starting to brown. Transfer the bacon to a paper towel covered plate to drain.
Remove all but 2-3 tablespoons of the bacon fat and reserve for another use. Return skillet to the heat, then add the onion, ½ tsp of the salt and the sugar, and saute until translucent, 5-7 minutes. Remove the skillet from the heat and allow to cool
Preheat the oven to 425ºF
In a bowl mix together the sour cream, heavy cream, and eggs, and then whisk together until combined. Add the remaining ½ tsp of salt, the nutmeg, and a few solid cranks of fresh ground pepper.
Butter a half baking sheet. Dump the risen dough out onto a surface lightly dusted with flour, and roll it out to a rectangle that is slightly larger than the half baking sheet. Press the dough into the baking sheet, letting the excess hang over the edges.
Spread the cooked onions evenly over the dough, then sprinkle the bacon on top of the onions. Carefully pour the sour cream mixture over the entire surface. Don't worry if it is too thick to fill all the nooks and crannies, it will spread out during baking. Once all the toppings and sour cream have been added, fold the dough over the edges to form the crust.
Bake the tart for 25-35 minutes until the custard is set, the dough is golden brown, and the top of the bacon and onion mixture is just starting to brown. Serve hot or at room temperature.