Grease 9 x 13 inch baking dish or line with parchment paper. Place bread cubes in bottom of dish.
In large skillet over medium, heat oil. Add onion and heat for 3 minutes. Add mushrooms and heat for another 3 minutes. Stir in bell pepper, tomato, oregano, and 1/4 tsp salt; heat for 3 minutes. Stir in spinach and heat until wilted. Stir in black beans.
In large bowl, whisk together eggs, half-and-half or milk, cheese, black pepper, and 1/4 tsp salt. Add vegetable mixture to egg mixture and stir to combine. Pour over bread cubes in baking dish. Refrigerate for at least 30 minutes and up to 24 hours.
Preheat oven to 375°F.
Bake casserole for 40 minutes, until edges are crisp and center of casserole is set.