Grease a 9-by 13-inch baking dish. In a large skillet, heat the olive oil over medium heat. Add the onions and cook, stirring, until lightly browned. Add the garlic and cook one minute longer. Stir in the tomatoes and mustard; cook until the tomatoes soften. Stir in the spinach, peppers and seasonings; cook 1 minute longer. Remove the skillet from the heat and cool for 10 minutes. Arrange half of the bread in the bottom of the prepared baking dish. Top half with the vegetable mixture, half the Asiago cheese and half of the provolone. Make another layer with the remaining bread, vegetables, and cheeses. In a large bowl, lightly beat together the milk and eggs; pour over the vegetable-cheese mixture. Cover tightly and refrigerate overnight. When ready to serve, heat oven to 350 degrees. Cook the strata, uncovered, until the center appears to set, 40 to 50 minutes. Serve immediately.
Makes 12 servings