How to Make It
Cook bacon in a large skillet over medium until crisp, about 6 minutes. Transfer bacon to a paper towel-lined plate, reserving 1 1/2 tablespoons drippings in skillet; discard any remaining drippings. Finely chop bacon; set aside.
Add onion and garlic to drippings in skillet over medium-high heat; cook, stirring occasionally, until onion is browned and tender, about 10 minutes. Add spinach; cook until spinach wilts, about 2 minutes, stirring constantly. Toss together chopped bacon, onion mixture, bread cubes, and cheese in a large bowl. Arrange mixture evenly in an 8-inch square glass or ceramic baking dish coated with cooking spray.
Combine milk, yogurt, eggs, egg whites, mustard, pepper, and salt in a large bowl; stir with a whisk until well combined. Pour evenly over bread mixture. Cover and chill 8 hours or overnight.
Preheat oven to 350°F. Uncover baking dish; let strata stand at room temperature as oven preheats. Bake strata in preheated oven until top of strata is browned and a knife inserted in center comes out clean, about 1 hour. Let strata stand for 5 minutes before serving.