Spinach, Bacon, and Gruyère Breakfast Strata

center-cut bacon slices

chopped yellow onion (about 1 medium onion)

garlic cloves, thinly sliced

fresh spinach, chopped

crusty whole-grain bread, cut into 1-in. cubes

cave-aged Gruyère cheese, shredded (3/4 cup)

Cooking spray

1% low-fat milk

plain 2% reduced-fat Greek yogurt

large eggs

large egg whites

Dijon mustard

freshly ground black pepper

kosher salt