4 large eggs
2 cups milk
1/2 teaspoon hot sauce
3/4 teaspoon salt, divided
1/4 teaspoon freshly ground pepper, divided
3 1/2 cups cubed stale whole-wheat country bread
1 10-ounce package frozen spinach
1 tablespoon extra-virgin olive oil
1 cup chopped onion (1 medium)
1 cup chopped red bell pepper (1 small)
4 cups sliced mushrooms (12 ounces)
1 clove garlic, minced
1 cup grated Gruyère, Emmentaler or manchego cheese (3 ounces), divided
Whisk the eggs in a large bowl. Add the milk, hot sauce, 1/2 teaspoon salt and 1/8 teaspoon pepper; whisk until blended. Add the bread and stir to coat well. Cover and refrigerate for 30 minutes.
Meanwhile, preheat the oven to 350 degrees F. Coat an 8 x 8-inch (or similar 8-cup) ceramic or glass baking dish with cooking spray. Put a kettle of water on to boil for the water bath.
Cook the spinach according to package directions. Drain, refresh with cold water and squeeze out excess moisture. Set aside.
Heat the oil in a large nonstick skillet over medium heat. Add the onion and bell pepper; cook, stirring often, until tender and golden, 4-5 minutes. Add the mushrooms and garlic; stir to blend. Reduce the heat to medium-low, cover and cook until tender, about 5 minutes. Uncover and increase heat to medium. Cook, stirring, until any excess moisture from the mushrooms has evaporated and mushrooms begin to brown, about 5 minutes. Add the spinach, remaining 1/4 teaspoon salt and 1/8 teaspoon pepper; cook, stirring, 2 minutes.
Add the mushroom mixture and 1/2 cup cheese to the egg mixture; stir until blended. Scrape into the prepared baking dish; sprinkle the remaining 1/2 cup cheese evenly over the top. Place the baking dish in a larger pan, put in the oven and pour boiling water into the larger pan to about halfway up the sides of the baking dish. Bake until the pudding is set in the center, 50-60 minutes. Let cool slightly; serve hot or warm.