1. Preheat oven to 425 degree F.
2. Divide and toss bell peppers, asparagus, and onion with oil on 2 large sheet pans; spread into a single layer. Roast in upper and lower thirds of oven, stirring and switching position f pans halfway through baking, until golden brown, about 15 minutes. Transfer pans to racks; cool to room temperature.
3. Lightly oil a shallow ovenproof 2-1/2-quart ceramic or glass baking dish. Whisk eggs, milk, cilantro, mustard, and salt together in a bowl until combined well.
4. Arrange half of bread in prepared dish and sprinkle evenly with half of roasted vegetables, then half of cheese. Repeat, layering with remaining bread, vegetables, and cheese. Pour egg mixture evenly over strata, cover with plastic wrap, and chill 6 hours.
5. Preheat oven to 350 degree F. Uncover strata; let stand at room temperature 20 minutes. Bake in middle of oven until puffed, golden brown and mixture is just set, 45 to 50 minutes. Let stand at least 10 minutes before serving. Strata will settle once removed from oven. Makes 6 to 8 servings.