12 ounces croissants (about 4 large or 8 mini), cut into small pieces
Everything But the Bagel Seasoning (optional)
Caramelize the onions; saute the mushrooms and spinach. Heat the olive oil over medium-low heat in a large skillet. Add the onions and cook, stirring occasionally, until the onions are jammy and rich golden brown in color, about 45-60 minutes. Push the onions to the side of the skillet, increase heat to medium and add the mushrooms. Saute the mushrooms until most of their water has evaporated, about 5-10 minutes. Fold in the spinach and gently stir to combine onions and mushrooms. Remove from heat when the spinach has wilted.
Make the egg mixture. While the onions caramelize, combine the eggs, dijon mustard, salt, and milk. Whisk until well combined.
Layer your ingredients. Preheat the oven to 350°F. Lightly grease a 9×13 baking dish with melted butter. Spread the veggies evenly over the bottom of the pan followed by half of the cheese. Pour eggs over the veggies and top with the croissant pieces. Finish with the remaining cheese.
Bake. Looselycover the pan with foil and bake until eggs are mostly set but still slightly jiggly in the center, about 30 minutes. Remove the foil and bake until the center is just set, about another 5-10 minutes. Top with Everything Bagel seasoning, if desired. Slice and serve!