Prep your ingredients. Dice the onion and bell pepper.
Slice the mushrooms if using. Mince the garlic. Shred the mozzarella if it’s not pre-shredded—freshly shredded melts better.
Brown the beef. Heat a large skillet over medium-high.
Add the ground beef and season with a pinch of salt and pepper. Cook, breaking it up with a spatula, until browned and no longer pink, about 5–7 minutes.
Drain excess fat if needed. If the beef releases a lot of grease, carefully spoon some off. Leave a bit for flavor—this helps the sauce cling to the meat.
Sauté the veggies. Push the beef to one side.
If the pan looks dry, add a splash of olive oil or butter. Add the onion, bell pepper, and mushrooms. Cook 3–4 minutes until softened.
Stir in the garlic for 30 seconds until fragrant.
Season like pizza. Sprinkle in the Italian seasoning and red pepper flakes. Stir to coat the meat and veggies so the spices bloom in the heat.
Add the sauce. Pour in the low-carb marinara. Stir until everything is coated.
Taste and adjust salt and pepper. Simmer for 2–3 minutes to meld the flavors.
Cheese it up. Reduce heat to low. Sprinkle mozzarella evenly over the skillet mixture, then dust with Parmesan.
Lay pepperoni slices on top if using.
Melt and finish. Cover the skillet with a lid for 2–3 minutes, until the cheese is gooey and melted. If you’d like browned, bubbly cheese, pop the skillet (oven-safe only) under the broiler for 1–2 minutes. Watch closely.
Garnish and serve. Top with olives and a sprinkle of chopped basil or parsley.
Spoon into bowls and enjoy immediately.