To make the crust: In a food processor, combine flour, salt and sugar. Pulse until incorporated. Add in pieces of salt until the mixture looks like crumbs, about 10 seconds. With the motor running, slowly add ice water, 1 tablespoon at a time, until dough forms together in a ball. You may not need all 3 tablespoons of water.
Shape dough in a ball, flatten into a disk, and cover with plastic wrap. Refrigerate for at least an hour before rolling out. Dough can be refrigerated overnight.
To make the filling: In a heavy bottomed skillet, heat butter and olive oil. Add onion, salt, and sugar and cook until onions start to brown, about 10 minutes. Stir in mushrooms and cook until they’ve released their juices, about 5 minutes. Stir in brussel sprouts. Cook for an additional 2-3 minutes, until brussel sprouts have turned bright green. Remove from heat and let cool. Adjust salt and pepper to taste.
In a medium sized bowl, combine ricotta, egg, Parmesan, and blue cheese. Mix in cooled vegetables.
Preheat oven to 400.
Roll out dough and press into a 9 inch pie pan. Spread filling evenly into dough. Sprinkle with additional blue cheese, if desired.
Bake for 30-35 minutes or until crust is golden brown. Let cool slightly before serving.