Preheat the oven to 375°F (190°C). Line a baking sheet with parchment paper.
In a mixing bowl, whisk the eggs with salt and pepper until well combined.
Heat olive oil in a skillet over medium heat. Add diced onion and bell peppers, sautéing until softened, about 3-4 minutes.
Pour the beaten eggs into the skillet and stir gently, cooking until just set, about 2-3 minutes. Stir in cooked sausage if using and remove from heat.
Toast the English muffin halves until golden brown, either in a toaster or on the stovetop.
Place the toasted English muffin halves on the prepared baking sheet. Spoon the egg mixture generously onto each half.
Top each muffin with shredded mozzarella cheese.
Bake in the preheated oven for 5-7 minutes, or until the cheese is melted and bubbly.
Remove from the oven, garnish with fresh basil or parsley if desired, and serve hot.