Preheat the oven to 375°F (190°C).
Place the English muffin halves on a baking sheet, cut side up. Toast them in the oven for 5 minutes until lightly crisp.
In a medium bowl, whisk together the eggs, whole milk, salt, and black pepper until fully combined.
Heat the unsalted butter in a non-stick skillet over medium heat. Pour in the egg mixture and cook, stirring gently, until the eggs are softly scrambled and just set, about 3-4 minutes. Remove from heat.
Remove the toasted English muffins from the oven. Evenly distribute the scrambled eggs over each muffin half.
Sprinkle shredded sharp cheddar cheese over the eggs on each muffin.
Top each muffin with crumbled breakfast sausage, chopped bacon, diced red bell pepper, and chopped green onions.
Sprinkle shredded mozzarella cheese evenly over all the toppings.
Return the baking sheet to the oven and bake for 7-8 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool for 2 minutes before serving.