Cook your potato skins if not done already (will add extra cook time). See notes for links to recipe for potato skin shells.
Cook the ground beef until just no longer pink. (It cooks a touch more in the oven later). Drain excess fat. Mix with the marinara sauce. Set aside.
Make the ricotta mixture by combining all of the ricotta mixture ingredients together in a small bowl. Set aside.
Preheat oven to 350F.
Once your potato skins are prepared, place them skin side down on a lined large cookie sheet.
Scoop approximately 1 1/2 Tablespoons of the ricotta cheese mixture into the bottom of each potato skin shell. Use just enough to cover most of the bottom in a thin layer. Repeat with all potato skins.
Add approximately 1 1/2 Tablespoons of the beef mixture into each potato skin shell.
Sprinkle each potato skin with the mozzarella and a little Parmesan.
Bake the potato skins in the oven just until the cheese is melted, approximately 5-7 minutes.