Preheat oven to 375°F. Lightly grease or spray with non-stick cooking spray a 9 x 13 baking pan.
Cook shells in large pot of boiling salted water for 9 minutes. Drain pasta.
In a medium bowl, combine ricotta cheese, spinach, red peppers, parmesan cheese and parsley. Salt and pepper filling mix to taste.
Spread 1-1/2 cups marina sauce in bottom of prepared pan. Stuff each shell with 2 tablespoons of filling. Place filled shells on top of marinara sauce in pan. You may need to scrunch in the shells. Spread remaining marinara sauce over the top. Sprinkle with mozzarella cheese.
Bake on center shelf of oven for 35 - 40 minutes until the sauce is bubbling and cheese is melted. Let stand 10 - 15 minutes before cutting and serving.
MAKE AHEAD FREEZER INSTRUCTIONS:
Prepare stuffed shells thru Step 4. DO NOT BAKE. Cover pan with aluminum foil, then wrap entire pan in plastic wrap or place in large freezer bag. Keeps in the freezer for up to 3 months. Thaw in the refrigerator overnight before baking, or bake right from the freezer.
To Bake Thawed Stuffed Shells
Remove plastic wrap and foil cover. Place uncovered in preheated 375°F oven. Bake for 40 - 45 minutes until sauce is bubbly and cheese is melted. Check the center of pan to make sure stuffed shells are heated thru. Continue baking and checking every 5 minutes until center is hot.
To Bake Frozen Stuffed Shells
Remove plastic wrap but leave foil cover. Preheat oven to 375°F. Bake covered for 1 to 1-1/2 hours. Remove foil cover and continue to bake for 20 to 30 minutes until the sauce is bubbling and the cheese is melted. Check the center of pan to make sure stuffed shells are heated thru. Continue baking and checking every 5 minutes until center is hot.