One day before preparing the bread pudding, place the bread cubes on a baking sheet and let stand at room temperature overnight or until the bread is dried out.
The next day, in a large skillet, cook the bacon over medium-high heat, turning as necessary, until evenly brown and crisp, about 5 minutes. Remove to paper towels to drain.
Butter a 9-by-13-inch baking dish. Arrange the bread in the dish. Scatter the bacon, peppers, and spinach leaves evenly over the cubes. Sprinkle with the 2 cups cheese. With a large spoon, evenly distribute the ingredients.
In a large bowl, whisk together, or beat with an electric mixer on medium speed, the eggs, milk, mustard, and salt and pepper to taste until well blended. Ladle the milk mixture over the bread cubes and gently press the bread into the liquid. Sprinkle with the remaining 1/4 cup cheese. Cover with aluminum foil and refrigerate overnight.
The next morning, remove the bread pudding from the refrigerator about 1 hour before baking and preheat the oven to 350°F (175°C). Bake the pudding for 45 to 50 minutes, or until slightly puffed, set, and browned on the top. If the center is still underdone, push the bread down with a wooden spoon to help the bread absorb the liquid. Bake for a few more minutes. Let rest for a few minutes and then cut into squares. Serve slightly warm or at room temperature. [To make the bread pudding in advance, bake it, let it cool, cover loosely, and then keep at room temperature for up to 4 hours. Reheat in a 350°F (175°C) oven for 15 minutes, if desired.]