Heat a large skillet over medium heat and add a splash of oil.

Sauté the chopped onion until soft, about 3–4 minutes. Add the minced garlic and cook 30 seconds until fragrant.

Add the ground beef. Break it up with a spatula and brown thoroughly, 6–8 minutes. Season with salt and pepper. Drain excess fat if the beef is very greasy.

Stir in the sliced mushrooms and cook 2–3 minutes until they start to soften.

Pour in the marinara sauce and mix until the beef is evenly coated. Simmer gently for 2–3 minutes to meld flavors. Taste and adjust seasoning; add oregano or red pepper flakes if desired.

Fold in most of the sliced pepperoni (reserve some for topping). Let everything warm together for another minute.

Meanwhile, split and toast the English muffins until golden. Preheat the oven broiler.

Spoon the beef-and-sauce mixture onto each muffin half. Top with shredded mozzarella and the reserved pepperoni slices.

Place on a baking sheet and broil 2–4 minutes, watching closely, until the cheese is melted and bubbling and the pepperoni edges crisp. Remove and let rest 1 minute before serving.