Begin by preheating your oven to 375°F (190°C) and lightly greasing a mini muffin tin.

Bring a large pot of water to a boil over high heat.

In a deep skillet, heat the ground beef over medium-high heat. Cook and crumble the meat until browned, about 5 to 7 minutes. Drain excess fat, then add the chopped onion and cook for 3 to 4 minutes until softened. Stir in the marinara sauce, 1 teaspoon of salt, pepper, garlic powder, and Italian seasoning. Allow the mixture to simmer, reduce the heat, and cook for 20 minutes, stirring occasionally.

Add the remaining 3 teaspoons of salt and pasta shells to the boiling water. Cook the pasta for 10 to 12 minutes, stirring occasionally, until tender. Drain the shells and rinse them under cold water to stop the cooking process. Drain well.

Carefully invert the pasta shells and arrange 24 shells in the prepared muffin tin. Reserve any broken or extra shells for future use.

Fill each shell with approximately 2 teaspoons of ricotta cheese, followed by a sprinkle of Parmesan cheese. Top with a tablespoon of the prepared meat sauce, then sprinkle evenly with mozzarella and the remaining Parmesan.

Place the muffin tin in the oven and bake for around 15 minutes, or until the shells are golden around the edges and the cheese is bubbly. Let the cups cool for 10 minutes before serving. Optionally, garnish with finely chopped parsley for a burst of freshness.