In a large pan heat the olive oil over medium heat. Sauté the onions and garlic until tender, about 3-4 minutes.
Add in the Italian seasoning (4 tsp), red pepper flakes (¾ tsp), Kosher salt (2 tsp), and black pepper (1 tsp). Stir in the tomato paste (3 Tbsp).
Pour in the canned tomatoes. Simmer for 30 minutes on low.
Make the Ricotta Cheese Mixture
Thaw the frozen spinach according to package directions. Place in strainer to cool and drain. Once cool enough to handle, use your hands to squeeze out as much water as possible from the spinach. Place the spinach in a bowl and chop roughly with kitchen shears.
Add in the Ricotta cheese, Parmesan cheese, eggs, and a pinch of kosher salt and black pepper. Mix well to combine.
Assembling the Lasagna
Preheat oven to 350°F. Grease a 9x13 casserole dish. Gather all of the ingredients near the dish to make a layering station.
Spread a thin layer of marinara sauce in the bottom of the pan. Place a layer of noodles on top of the sauce. Don't worry if they don't fill the entire pan exactly. Four sheets should fit in the bottom of a 9x13 pan.
Place a dollop of ricotta cheese mixture (~¼ C) on each noodle and spread it out. Then sprinkle with mozzarella cheese (~⅓ C). Spread about 1 cup of marinara sauce on top of the cheeses. Place another layer of noodles on top of the sauce. Repeat this step 2 more times.
On top of the last layer of noodles, spread another cup of sauce. Sprinkle with mozzarella cheese.
Cover the lasagna with foil. Bake at 350° for 60-80 minutes. Remove the foil for the last 15-20 minutes to brown the cheese.
For Meat Lasagna: If you want meat lasagna, first cook and crumble 2 pounds of Italian Sausage, drain the fat and set the cooked sausage aside. Add the sausage back into the sauce during the simmer.
Cook Time: Depending on the size pan you use or if the lasagna was cold (refrigerated), it may take less/more time to cook. Keep an eye on it. It's done when the sauce is bubbly around the edges, the cheese is melted on top and slightly brown, and the center is about 165°F.
Freezing: Freeze uncooked, wrapped tightly. Thaw completely (at least overnight) in the fridge before baking.