2 tablespoons olive oil
Half an onion, diced
2 bell peppers (any color), diced
1 cup sliced mushrooms
1 garlic clove, minced
1 cup fresh spinach
Kosher salt
Freshly ground black pepper
Butter for greasing
4 slices day-old sourdough bread, cut into cubes
10 large eggs
1/2 cup milk
1 cup shredded sharp cheddar cheese, divided
In a large skillet over medium-high heat, heat the olive oil.
Add the onion, peppers, mushrooms and garlic and cook, stirring frequently, until the veggies have softened, about 8 minutes.
Add the spinach and cook a couple of minutes more, until wilted. Season to taste with salt and pepper.
Remove from the heat and let cool for several minutes.
Generously grease a 9-by-13-inch baking dish with butter. Arrange the bread cubes in the bottom of the baking dish in an even layer.
In a large mixing bowl, whisk together the eggs, milk and 1/2 cup of the cheese. Stir in the vegetable mixture and pour the whole thing over the bread in the baking dish.
Season with a bit more salt and pepper, if desired. Sprinkle the remaining 1/2 cup cheese over the top.
Cover and refrigerate for at least 2 hours (or as long as overnight) before baking.
Preheat the oven to 375 degrees F. Let the dish come to room temperature while the oven heats.
Bake the casserole, uncovered, for about 45 minutes, until the center is set and the top is golden brown.
Serve warm out of the oven or at room temperature.
