Brown: In a large pot/pan, brown beef and onion on Level 7 (Medium-High). Drain fat.
Pasta: Add the pasta shells and enough water (or broth) to just cover the pasta (about 2-3 cups). Stir in the heavy cream (and marinara if using).
Simmer: Bring to a boil, then reduce to Level 4 (Medium-Low). Cover and simmer for 10-12 minutes until pasta is tender and liquid is mostly absorbed.
Cheese: Stir in the shredded cheddar (and the cheese packet if using boxed mac) and frozen peas until melted and creamy.