Grease one large deep dish pie pan.
Prepare crust: Combine the first 4 ingredients in the food processor and pulse to mix.
Add the cut butter pieces and pulse a few times through until butter is the size of peas and ingredients start to clump and hold together.
In a small bowl, whisk together the egg yolk and 3 tablespoons ice water with a fork.
Add it to the dough slowly while pulsing until a dough ball forms. Do not over pulse; stop as soon as it begins to clump and there are nearly no flour bits left. Add more ice water only if needed (dependent upon your elevation and the humidity levels in your kitchen). *You’ll want enough moisture in the dough to prevent cracking, so add water until it’s slightly sticky on your fingers.
Remove dough from bowl and place on lightly floured surface.Work gently into ball. Flatten to 1” disk. Wrap in plastic, chill in fridge for 20-30 minutes.
When chilled, roll the dough to about 1/8-inch thickness. Fill the pie pan(s) with the dough and cut off excess. Alleviate any cracks by pressing into place.
Place in freezer while you make the filling. This will help keep its shape during baking.
Prepare caramelized onions: In a large Dutch oven, heat 3 tablespoons olive oil over medium to medium high heat. Add half of the onions and cook, stirring occasionally for 3 minutes.
Add remaining onions. Season with salt and pepper and toss. Continue to cook, stirring occasionally, about every 3-5 minutes, until onions begin to stick to bottom of pot, stirring up any browned bits from bottom for about 12-15 minutes.