Cube the stick of butter and add to food processor (I miss using my hands to warm up the cold butter cubes,but I admit this way is very quick! If not using a processor, add the cubes to a mixing bowl.) Add flour and salt, and pulse about 12 times. Add cold water and pulse until the texture looks like sand.
Empty out onto a flat counter surface and gently knead just a little bit to incorporate the ingredients, but don't over knead. Roll into a ball, press into a disc and wrap in cling film.Refrigerate for 15-30 minutes.
Once chilled, roll the dough with a floured rolling pin onto a floured surface, making sure to flip over occasionally to prevent sticking until the circle is at least 1-2 inches larger than the diameter of your tart pan.
Lay the dough over the tart pan (I like to butter the tart pan for extra anti-stick protection and buttery-ness of course), pressing into the ridges with a small piece of excess dough. Trim off the tops of the ridges/pan with a knife, making sure to slice away from you.
Prick the bottom of the tart pan dough all over with a fork. Refrigerate for 2 hours or freeze (I froze overnight in order to make for breakfast.) Note: my overnight frozen tart receded a bit from the top of the shell.
Preheat oven to 400ºF. Fill the tart shell with beans after placing a piece of parchment paper into the shell. Blind bake for 15 minutes. Remove the beans and continue to bake for another 5-10 minutes, until the bottom is dry and lightly golden brown.
Saute the sliced onions over medium heat into a couple tablespoons of olive oil and (Kerry Irish) butter. Stir continuously for 30 minutes or more until they caramelize to your liking.
Whisk the eggs & milk, salt, pepper, nutmeg and thyme. Add in the cooled onions.
Add in 3/4 of the shredded cheese. Pour into tart pan, making sure to have pan set onto a baking sheet. (Make sure to discard any scrambled egg extras that arise from an overfilled base!)
Bake for 35-40 minutes (lower to 375 degrees if browning too quickly.) Let sit 5-10 minutes before serving. Enjoy whatever combination of cheese, egg, and onion you have. This is a simple, yet rich recipe. Add in any additional flavors,like sausage, ham, bacon, tomato, olives, zucchini. The tart's the limit.