Preheat oven to 325F
Add olive oil and onions to a large oven safe skillet over medium heat. Cook until brown and caramelized, approx. 10 minutes.
In a separate skillet, cook bacon over medium heat until brown and crispy, approx. 10 minutes.
Drain excess fat and add to skillet with onions, turn heat down to low.
Whisk eggs and milk together until smooth.
Spread bacon and onions evenly in pan and cover with cheese. Pour eggs slowly over.
Season with salt, pepper and thyme.
Place skillet in oven and bake for 30 minutes.
Remove skillet from oven and allow to rest for 10 minutes.
Using a rubber spatula, peel sides of frittata away from skillet.
Place a large cutting board over skillet and flip.
Cut into wedges and enjoy!