Add the ricotta, mozzarella, parmesan, and egg to a large mixing bowl. Using a spoon or spatula, mix gently until everything is evenly combined. Take your time and avoid overmixing — the goal is a smooth, creamy filling that stays light and fluffy rather than dense.

Fold in the cooked protein and seasonings using slow, gentle motions. Make sure the protein is evenly distributed throughout the cheese mixture without breaking it down too much. This ensures every shell has a balanced mix of cheesy creaminess and savory filling.

Lightly grease a baking dish, then spread a thin, even layer of marinara sauce across the bottom. This base layer prevents the shells from sticking, keeps them moist during baking, and adds flavor from the very first bite.

Using a spoon or piping tool, carefully stuff each shell with the prepared cheese mixture. Fill them generously but without forcing the mixture, so the shells hold their shape. Arrange the stuffed shells snugly in the baking dish, side by side, so they support each other as they bake.

Spoon the remaining marinara sauce over the stuffed shells, allowing some of the filling to remain visible. This keeps the dish from becoming too saucy while still adding richness. Sprinkle shredded cheese evenly over the top without pressing it down, so it melts smoothly and creates a bubbly, golden finish.

Place the baking dish into the preheated oven and bake until the sauce is gently bubbling around the edges and the cheese is fully melted and lightly golden. The shells should be heated through and the filling set but still creamy.

Remove the dish from the oven and let it rest for a few minutes before serving. This short resting time allows the filling to firm up slightly, making the shells easier to serve and helping the flavors settle for the best texture and taste.