Boil the noodles: Cook lasagna sheets according to package directions until al dente. Drain and separate so they don’t stick. Lay them flat on a lightly oiled tray or clean towel.

Make the meat sauce: In a large skillet over medium heat, add chopped onion and sausage. Cook, stirring occasionally, until the sausage is fully cooked and the onions soften (about 6–8 minutes). Add minced garlic, dried parsley, basil, oregano, and the tomato sauce. Stir, reduce heat to low, and simmer 10–15 minutes. Remove from heat and let cool slightly.

Prep the ricotta filling: In a bowl, combine the ricotta, egg, 1/4 cup Parmesan, 1 cup mozzarella, and chopped spinach. Mix until smooth and evenly distributed. Season with a little salt and pepper to taste.

Spread sauce in dish: Spoon about 1/4 cup of the meat sauce into the bottom of a 9x13 baking dish to prevent sticking.

Assemble rolls: Lay a lasagna sheet flat. Spread ~2 tablespoons of the ricotta mixture down the center of the noodle, leaving a small border. Spoon ~2 tablespoons of meat sauce on top of the ricotta. Starting at one end, roll the noodle tightly and place seam-side down in the prepared baking dish. Repeat with remaining noodles (you should have 12 rolls).

Top and bake: Pour remaining meat sauce evenly over the rolls. Sprinkle with the remaining 1 cup mozzarella and remaining Parmesan. Cover with foil and bake at 350°F for 40 minutes. Remove foil and bake 5 more minutes to brown the cheese. Let cool 5–10 minutes before serving.