Grease a 9x13-inch baking dish with non-stick spray or butter. Cut the day-old bread into bite-sized cubes and layer them evenly in the dish.

In a large skillet over medium heat, brown the breakfast sausage or bacon until fully cooked and crispy, about 5-7 minutes. Drain excess grease, then add diced onion and sauté for an additional 3-4 minutes.

Evenly distribute the cooked sausage and sautéed onion over the bread cubes. Sprinkle in diced bell peppers or any other vegetables, ensuring an even mixture.

Whisk together eggs, whole milk, and heavy cream in a large mixing bowl until smooth. Add salt and black pepper to taste, blending into the custard.

Pour the custard mixture evenly over the bread and filling layers, gently pressing down on the bread. Sprinkle shredded Cheddar and optional Parmesan cheese on top. Cover with plastic wrap and refrigerate overnight.

Preheat the oven to 350°F (175°C). Remove casserole from the fridge and uncover it. Bake for about 45-50 minutes or until golden brown and custard is set.

Let the casserole cool for about 5-10 minutes before slicing. Garnish with fresh parsley before serving.