Heat the olive oil in a large oven-safe skillet over medium-heat. Stir in the onions with a big pinch of salt. Cook over medium, stirring often, until the onions soften, about 10 to 15 minutes.

Decrease the heat to low or medium-low. Cook the onions, stirring often, for about 60 to 90 minutes, until they are golden and caramelly. If at any time the onions start to burn, low the heat or add a few splashes of water to keep them hydrated.

Once the onions are golden and caramelly, transfer them to a plate. (Note: if you’re really in a rush, you can add 1 teaspoon of sugar, honey, maple syrup, etc to speed along the caramelization process. Be sure to stir often and check on the progress!)

While the onions are cooking, preheat the oven to 450 degrees F.

Place the croissants cut-side up on a baking sheet. Bake for 5 to 8 minutes, until the croissants are toasty. Remove and let them cool slightly, then chop or tear them into pieces.

Brush a 9x13 baking dish with melted butter. Place the croissants in the dish and toss them with half of the caramelized onions. Be sure to toss well so the onions are distributed evenly.

In a large bowl, whisk together the eggs, milk, heavy cream and a big pinch of salt and pepper. Stir in 1 cup of the gruyere cheese. I also like to stir in a handful of fresh chopped herbs, like parsley and thyme, but it’s up to you! Place the rest of the cheese in a bag stored in the fridge.

Pour the egg mixture over the top of the croissant mixture in the baking dish. Press any of the croissants into the egg liquid, making sure they are covered and absorbed the eggs. Top with the remaining caramelized onions.

Cover the dish with plastic wrap and refrigerate 8 hours, or overnight.

Preheat the oven to 350 degrees F. Remove the dish 30 minutes before baking to take the chill off.

Remove the plastic wrap. Top with remaining grated cheese stored in the fridge. Bake the dish uncovered for 45 to 60 minutes, or until the center is set. Top with some parmesan and a sprinkle of fresh thyme before serving. Serve!