Heat the olive oil in a skillet over medium heat. Cook the pancetta, stirring occasionally, for 5 minutes, until just crisp. Remove pancetta from the skillet but leave the grease.
Sauté the onion in the grease for about 7 minutes. Add the garlic to the sautéed onion and cook for 1 minute more.
Combine the cooked garlic and onions with the pancetta.
Mix the eggs, milk, mustard, thyme, and salt in a separate bowl, then stir the egg mixture into the pancetta.
Stir in 1 cup of cheese.
Lightly grease an 11x7-inch baking dish. Tear the croissants into pieces and arrange the pieces in the bottom of the dish.
Pour the egg mixture over the croissants. Sprinkle the remaining cheese over the casserole.
Let the casserole sit for at least 1 hour before baking.
Preheat the oven to 350 F.
Bake the casserole for 50 to 55 minutes, checking halfway through. If it's already browning, cover it with foil for the final 20 minutes of baking.
Remove the casserole from the oven when it's set and serve it warm.