Overnight Egg and Breakfast Sausage Strata

INGREDIENTS

1 lb

bulk ground breakfast sausage (See Kelly's Notes)

1 tbsp

unsalted butter, plus more for greasing pan

1 cup

diced onion

large eggs

2 1/2 cups

whole or non-fat milk

2 tsp

Dijon mustard

1/2 tsp

salt

1/4 tsp

black pepper

1 cup

shredded cheddar cheese

asparagus spears, ends discarded and spears cut into 1-inch pieces

1/2 lb

white, whole wheat or French bread, cut into 1-inch cubes (about 7 cups)