Preheat your oven to 375°F (190°C).
Place the English muffin halves on a baking sheet, cut side up. Drizzle with olive oil and lightly toast in the oven for about 5 minutes.
Remove the muffins from the oven and spread about 2 tablespoons of marinara sauce on each half.
Sprinkle the shredded mozzarella cheese evenly over the sauce.
Top each muffin with crumbled breakfast sausage, bell peppers, and mushrooms.
Season with garlic powder, Italian seasoning, salt, and pepper.
Return the baking sheet to the oven and bake for an additional 10-12 minutes, or until the cheese is melted and bubbly.
Remove from the oven and let cool for a minute. Garnish with fresh basil leaves if desired.