1In a large skillet, heat 1 teaspoon oil over medium-high heat. Add mushrooms; cook and stir until tender, 8-10 minutes. Remove from pan.
2In same pan, heat 1 teaspoon oil over medium heat. Add onion; cook and stir until golden brown, 6-8 minutes. Remove from pan and add to the mushrooms.
3Add remaining oil to pan. Add peppers, leek, salt and pepper; cook and stir until leek is tender, 6-8 minutes. Stir in sautéed mushrooms and onion.
4In a 13x9-in. baking dish coated with cooking spray, layer half of each of the following: bread cubes, vegetable mixture, mozzarella cheese and Parmesan cheese.
5Repeat layers. In a large bowl, whisk eggs, egg whites and milk until blended; pour over layers. Sprinkle with basil.
6Refrigerate, covered, overnight.
7Preheat oven to 350°F. Remove vegetable strata from refrigerator while oven heats.
8Bake, covered, 50 minutes. Bake, uncovered, until lightly browned and a knife inserted in the center comes out clean, 10-15 minutes longer. Let stand 10 minutes before serving.