This is a great way to use leftover bread, while also making an easy and delicious brunch party dish.
Technique tip: Check it 30 minutes into cooking and if the top is already nicely browned, then cover it with aluminum foil to avoid over-browning.
Swap option: You can swap Comté cheese for Gruyère.
Pour the olive oil in a cast-iron skillet over medium heat. Add the sliced onions and a couple pinches of salt, and stir to coat the onions completely in the olive oil. Turn the heat to low, and it'll stay on low for the rest of the time you're caramelizing the onions. Keep a close eye on the onions, with frequent stirring.2.
As you continue to frequently stir the onions, you'll notice them first go transparent, then begin to brown. You're doing good, keep stirring over low heat. Once they've been on the heat for around 10-15 minutes, you can add a dollop of salted butter, the sliced mushroom and another pinch of salt and some black pepper, and stir. Let cook for 2 minutes, stirring frequently.3.
Add the thyme leaves to the pan, stir and take the pan off the heat to let cool slightly.4.
Prepare the baking dish: I use an 8-inch glass baking dish for this, because I like to see the layers of the Strata and that size dish works well for this quantity of ingredients. Place the bread cubes into the dish so that they cover the bottom of the dish in one layer without overlapping. You'll have extra bread cubes — hang on to them to add them to the top of the dish just before putting it in the oven. Now add the onion and mushroom mixture on top of the bread cubes, spreading it as much as you can in one even layer. Then, add half of the cubed or grated Gruyère cheese in one even line on top of the vegetables.5.
In a mixing bowl, whisk together the eggs, milk and another pinch of salt. Pour this over the ingredients in the baking dish, then top with the other half of cheese. Cover the baking dish in plastic wrap and place it in the refrigerator for 8 hours (or overnight).6.
The next day, once the ingredients have all become friendly in the fridge, it's time to bake! Heat your oven to 350 F. Remove the plastic wrap from the baking dish and put it on the middle rack of your oven. Bake it for 45 minutes to an hour, until the egg is baked through and the top is browned.7.
Remove from the oven and let cool, then garnish with thyme leaves, cut into it and enjoy warm or at room temperature.