Prep the oven and pan: Preheat the oven to 400°F (200°C). Spread 1 cup of marinara sauce on the bottom of a 9x13-inch baking dish.
Slice the zucchini: Trim the ends.
Using a mandoline or a sharp knife, slice lengthwise into thin strips, about 1/8-inch thick. Aim for long, even slices that roll easily.
Draw out moisture: Lay slices on a sheet pan or clean towels. Sprinkle lightly with salt and let sit 10–15 minutes.
Pat dry on both sides to remove excess moisture.
Optional quick pre-bake: For sturdier rolls, brush slices with olive oil and bake on a lined sheet at 400°F for 6–8 minutes until pliable. Pat dry again. This step helps prevent watery rolls.
Make the filling: In a bowl, mix ricotta, 1/2 cup mozzarella, Parmesan, egg, garlic, Italian seasoning, red pepper flakes (if using), salt, and black pepper.
Fold in spinach, mushrooms, or cooked meat if using.
Assemble the rolls: Place a zucchini slice on a board. Spread 1–2 tablespoons of filling along the length, leaving a small border. Roll up snugly (not too tight) and place seam-side down in the sauced baking dish.
Repeat with remaining slices and filling.
Top and bake: Spoon remaining marinara over the rolls. Sprinkle with the remaining 1/2 cup mozzarella. Bake uncovered for 18–22 minutes, until bubbly and the cheese is melted.
Finish and serve: Let rest 5–10 minutes to set.
Garnish with chopped basil or parsley. Serve warm with a side salad or garlic bread.