Preheat oven to 375 degrees F and spray an 8x8-inch baking dish with nonstick cooking spray.
Spread the cubed bread evenly in the baking dish.
Sprinkle with spinach, bell peppers, and onion evenly over the bread.
In a large bowl, whisk together the eggs, almond milk, garlic powder, onion powder, salt, and pepper.
Pour the egg mixture evenly over the bread and vegetables, gently pressing down so everything is coated.
Sprinkle the reduced-fat cheddar and part-skim mozzarella cheese over the top.
Bake uncovered for 30-35 minutes, or until the center is set and the top is lightly golden.
Let the strata rest for about 5 minutes before slicing and serving.