Cook Sausage & Veggies: In a large skillet over medium-high heat, cook the ground sausage and diced onion until the sausage is browned and the onion is translucent, about 7-10 minutes. Add the minced garlic and cook for 1 more minute until fragrant. If using spinach, add it now and cook until wilted. Drain any excess grease from the skillet.

Layer the Casserole: Grease a 9x13 inch baking dish. Spread the bread cubes in an even layer at the bottom. Top with the cooked sausage mixture. Sprinkle the shredded cheddar cheese evenly over the sausage.

Make Custard: In a large bowl, whisk together the eggs, milk, dry mustard powder, salt, and pepper until completely smooth.

Soak & Chill: Pour the egg mixture evenly over the entire casserole, ensuring all the bread is moistened. Cover the dish tightly with foil and refrigerate for at least 8 hours or overnight.

Preheat Oven: Preheat your oven to 350°F (175°C). Remove the casserole from the refrigerator and let it sit on the counter for 20-30 minutes.

Bake: Remove the foil and bake for 50-60 minutes, until the center is set (a knife inserted into the center comes out clean) and the top is golden brown and puffed.

Rest & Serve: Let the casserole rest for 10 minutes before slicing and serving. Garnish with fresh chives or parsley if desired.