Gather all your ingredients. Preheat the oven to 400ºF.
If making homemade marinara (optional), start it first so it can simmer while you prep the roll ups: Sauté onion, garlic, and olive oil over medium until soft (5 min). Stir in tomato paste and cook until darkened (3-5 min). Add crushed tomatoes, basil, oregano, brown sugar, and salt, then bring to a simmer. Reduce to medium-low, partially cover, and simmer for 30 min, stirring occasionally. Taste and adjust seasonings (simmer longer or add more sugar if too acidic).
To make the rolls ups: Bring a large pot of water to a boil over high heat. Once boiling, add 1 tsp salt to season the water, then add the lasagna noodles. Boil the noodles just until al dente (they become too delicate to roll if they are too soft). Drain the noodles in a colander.
While the noodles are boiling, drain the thawed spinach, squeezing out as much of the excess water as possible. Add the spinach to a bowl with the ricotta, 1 cup of the mozzarella, Parmesan, egg, salt, and pepper. Stir until everything is evenly combined.
Once the lasagna noodles have cooled enough to touch with your hands, lay a few noodles out onto a clean surface. Spread about ¼ cup of the spinach and cheese filling onto each noodle, spreading it from edge to edge. It's okay if the filling is thin or doesn't create a solid layer. Roll the noodles up, then place in a large casserole dish. Repeat with the remaining noodles until you've used all of the filling. You should get about 16 roll ups and you may have a few noodles left over (those will be back up in case any tear).
Once all of the noodles are filled, rolled, and placed in the casserole dish, pour the pasta sauce over top. Top the lasagna roll ups with the remaining 1 cup of mozzarella cheese.
Cover the casserole dish with foil and bake the lasagna roll ups for 30 minutes in the preheated 400ºF oven. Serve hot and enjoy!
*Each lasagna noodle makes one bundle. Two rolls is one serving. I like to get the flutter-edge noodles instead of flat noodles because they make the prettiest roll ups.**I use roughly 1 cup of frozen spinach (once thawed and squeezed), but fresh spinach will also work. If using fresh, sauté it in a little oil until very wilted and most of the moisture is cooked out. A 10 oz. bag of fresh spinach should work.***You can also use a 23-oz jar of pasta sauce if preferred!If making the homemade marinara, you will need to measure out 23 oz. (approx 3 cups) of sauce once made, then store the rest for another meal (keep it in the fridge for 4-5 days or freeze for up to 3 months). Or you can justuse the whole batch of homemade marinara for some extra saucy lasagna roll ups!