1 onion, sliced intos trips
3 Tbsp butter, divided (or cooking oil of choice)
4 slices bacon, cut into 1/2" chunks
4-7oz mushrooms of choice, sliced
4 eggs
1 cup cream
1 cup + cheese of choice (mozzarella, gruyere, cheddar, parm, mixed if you like)
salt & pepper
optional: thyme, parsley

Heat oven to 350
Cook onions in 2 Tbsp butter or oil over low heat, covered, stirring every 5 minutes for ~20-25 minutes until well softened and ideally starting to brown a little.
Raise the heat a bit to medium-low. Move the onions aside and add bacon chunks in with onions and cook until bacon is cooked, but not crisping. Meanwhile, continue to stir and move the onions on their side of the pan, letting bacon fat render and add to the onions, which should continue cooking and begin lightly browning and caramelizing.
In another pan, melt 1 Tbsp butter. Add mushrooms and cook through until lightly browning. Salt and pepper as you like.
Whisk eggs and cream together. Add cheese and mix thoroughly, salt and pepper, season with thyme if you like.
Drain mushrooms, onions, and bacon, then mix and put into 9" pie plate, distributing evenly at bottom.
Gently pour egg and cheese mixture over the top. Sprinkle top with some grated parmesan if desired.
Bake for 45 minutes or until golden brown and knife in the middle comes out clean.