In a large saucepan over medium-high heat, brown the sausage and ground beef until fully cooked. Drain excess fat if needed, then return the meat to the pan. Add garlic-infused oil (or olive oil + garlic) and onion; sauté until soft. Stir in tomato paste and let it darken slightly, then add the crushed tomatoes, basil, oregano, salt, and pepper. Lower heat and simmer for about 30 minutes until sauce thickens slightly.
Heat olive oil in a medium saucepan and wilt spinach in batches (about ¼ at a time). If it’s drying, add a teaspoon of water—but don’t overdo it, spinach releases a lot of moisture itself. Let spinach cool, then transfer to a food processor. Add ricotta, Parmesan, eggs, salt, and red pepper flakes; blend until smooth and light green.
In the casserole dish, start with a thin layer of meat sauce (to prevent sticking). Lay 3 no-boil noodles (break pieces to fit edges), then spread half the meat sauce over noodles and sprinkle with ¼ of the mozzarella and parmesan. Next layer with noodles again, then the spinach-ricotta filling, and another ¼ of cheese. Repeat layering until you’ve used 4 alternating layers, finishing with a cheese topping. Cover with foil and bake at 375 °F (190–200 °C) for most of the baking time, then remove foil in the final 10 minutes to brown the top. Let rest 10–15 minutes before slicing and serving.