In a large skillet, heat 1 teaspoon oil over medium-high heat. Add mushrooms; cook and stir until tender, 8-10 minutes. Remove from pan., In 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., Add remaining oil to pan. Add peppers, leek, salt and pepper; cook and stir until leek is tender, 6-8 minutes. Stir in sauteed mushrooms and onion., In a 13x9-in. baking dish coated with cooking spray, layer half each of the following: bread pieces, vegetable mixture, mozzarella cheese and Parmesan cheese. Repeat layers. In a large bowl, whisk eggs, egg whites and milk until blended; pour over layers. Sprinkle with basil. Refrigerate, covered, overnight., Preheat oven to 350°. Remove strata from refrigerator while oven heats., Bake, 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.