French Onion Soup Breakfast Strata

Preheat oven to 350°F (175°C). Grease a 9x13 inch baking dish with 1 tablespoon of butter.

In a large skillet, melt 2 tablespoons of butter and olive oil over medium heat. Add the sliced onions and cook, stirring occasionally, until deeply caramelized, about 30-40 minutes. Stir in thyme and sherry (if using) during the last few minutes of cooking.

In a large bowl, whisk together the eggs, heavy cream, milk, Dijon mustard, salt, pepper, and nutmeg.

Arrange half of the cubed bread in the prepared baking dish. Top with half of the caramelized onions and half of the Gruyere cheese. Repeat layers with remaining bread, onions, and cheese.

Pour the custard mixture evenly over the bread and cheese layers. Gently press down on the bread to ensure it's submerged in the custard.

Let the strata sit for at least 30 minutes to allow the bread to absorb the custard (or refrigerate overnight for a more intense flavor).

Bake in the preheated oven for 35-45 minutes, or until golden brown and the custard is set. A knife inserted into the center should come out clean.

Let the strata cool for a few minutes before serving warm.