Step 1: Onions ... Add the butter in a large saute pan and bring to medium heat; then, add the onions and thyme springs. Cook the onions on medium heat for a good 5 minutes, then add a pinch of both salt and pepper. Continue to cook another 25-35 minutes on medium to medium low heat, until the onions are golden to dark brown, stirring often. Don't try to rush this process - you don't want them to brown too fast. Once the onions are tender and a nice dark caramel color. Remove from the heat, pull the sprigs of thyme out, stir in the creme fraiche; and season once again with salt and pepper.
Step 2: Pie Crust ... Roll out the dough on a sheet of parchment paper to approximately 12". This is a free form tart, so it doesn't have to be perfect. Transfer the parchment paper to a sheet pan.
Step 3: Tart ... Add the onions to the center of the pie crust about 1 1/2" from the edge. Fold the edge over the onions. Remember, this is a rustic tart, so it doesn't have to be perfect; you can really do it however you want. Then, mix the egg and milk together and brush the edges of the pie crust. This will help to create a nice brown crust. Sprinkle a little sea salt or kosher salt on the crust where you bushed with egg.
Step 4: Note: Sea salt is very flaky and delicate, a bit different than kosher salt. I love to keep some on hand to garnish dishes, but it isn't necessary. A pinch of kosher salt will work just fine.
Step 5: Bake ... Middle shelf in a 375 degree oven for 40-45 minutes, until the pie crust is golden brown - baking times can vary. Remember, any time you bake, I like to check the dish about 5 minutes before the suggested time - then, keep an eye on it. The top and bottom of the crust should be golden brown.
Step 6: Serve ... Cut into wedges and serve warm or room temperature. Add a salad or soup for a light dinner or lunch. These also make excellent appetizers. ENJOY!
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