Cook the bacon in a skillet until crisp. Drain it on paper towels and cool completely. Blot the diced tomatoes dry.
Butter a 9-by-13-inch dish. Layer the sourdough, bacon, tomatoes and cheese evenly.
Whisk the eggs, milk, salt, pepper and onion powder. Pour over the bread and press the top gently into the custard.
Cover and refrigerate for 8 to 16 hours. Set the dish out for 30 minutes while the oven heats to 350°F.
Bake uncovered for 50 to 55 minutes until the center is puffed and reaches 160°F. Rest 10 minutes before slicing.