Chrissy Teigen's Everything Bagel Cream Cheese Breakfast Bake

INGREDIENTS

1 lb

uncooked breakfast sausage links (optional, see Recipe Notes)

2 tbsp

unsalted butter, plus more for the baking dish

large onion, thinly sliced

1 10 ounce package

frozen spinach, thawed

day-old everything bagels, halved and cut into big chunks

large tomatoes, cut into chunks

1/4 cup

chopped fresh basil leaves

1 cup

finely grated Parmigiano-Reggiano cheese

1 1/2 cups

grated Gruyère or Swiss cheese, divided

1 1/2 cups

grated cheddar cheese, divided

2 tsp

kosher salt, divided

1 tsp

freshly ground black pepper, divided

large eggs

2 cups

milk

1 cup

half-and-half

2 tbsp

Dijon mustard

1/4 tsp

cayenne pepper

8 oz

cold cream cheese, cut into 14 pieces