1) Get milk lukewarm (105-115 degrees) and mix in sugar and yeast. Let it sit for 5 minutes until the yeast begins to foam.
2) Once it’s foaming, stir in egg, melted butter, salt, and flour. Stir everything until it comes together in a ball. If you are using a mixer, mix this on medium with the paddle attachment for 3 minutes or so. If you are using your hands then knead this on a clean surface for about 5 minutes.
3) Sprinkle a bit of flour on top and return it to the bowl to rise for 2 hours or until it doubles in size. To help it rise, cover the bowl with a damp, warm towel and keep it in a warm place.
4) For filling, half each onion, stem to stem, and then slice each half into narrow strips. Chop bacon into tiny pieces.
5) In a good sized pot or pan with a lid, add butter to the pan over medium heat and then add bacon. Cook bacon for a few minutes until it is translucent (not crispy or it will burn later). Then add all onions at once. Cook this, covered, for 20 minutes. Stir it every 5 minutes or so just to make sure everything is cooking evenly.
6) After 20 minutes of covered cooking there will be a lot of liquid in the pan to get rid of. Cook the onions for another 30 minutes uncovered. Give them a stir every 5 minutes or so.
7) Let onions cool down to room temperature for about 30 minutes.
8) In a large bowl, whisk together sour cream and egg yolks. Once onion mixture is cooled, add and stir it all together.
9) Season filling with a good pinch of salt and pepper.
10) On a lightly floured surface roll dough out to a large 15 inch circle.
11) Transfer your crust to a springform pan. Gently press the crust around all the edges.
12) Pour in filling and fold crust over the top. There should be a good amount of filling still showing in the center. You don’t want it completely covered.
13) Bake for about 80 minutes at 375 degrees.
14) Let it cool for 15 minutes before cutting (if you have a springform pan you can take the pie out though as soon as you can handle it).
15) You can also store the pie in the fridge for a few days and just reheat it for about 30 minutes at 350 in the oven or (heaven forbid) you can reheat it a slice at a time in the microwave!