Begin by cubing the whole wheat bread into ½” pieces. In a large bowl, toss the bread cubes with sliced breakfast sausage links and fresh spinach until evenly coated. Transfer this mixture to a resealable container for up to 4 days.

Preheat your oven to 350°F (175°C). Cube the bread into ½” pieces again if preparing fresh and slice the sausage links into ¼” coins.

In a large mixing bowl, combine the cubed bread, sausage pieces, and fresh spinach. Spread the combination evenly in a greased 9x13 inch casserole dish.

In a medium-size bowl, whisk together the mustard powder, salt, half and half, and eggs until fully blended. Aim for a pale yellow color.

Pour the egg mixture over the bread and spinach combination in your greased casserole dish. Stir together gently.

Sprinkle the shredded cheddar cheese evenly on top of the casserole mixture. Place the casserole in your preheated oven and bake for 25 to 30 minutes.