In a large bowl, mix together diced tomatoes, marinara sauce, and tomato paste. Set aside.

Heat oil in a large nonstick skillet over medium heat. Add onions, mushrooms, and garlic and sauté, stirring frequently, for 3 minutes. Add spinach and sauté, stirring frequently, until the liquid has cooked off, about 3 minutes more. Remove from heat and place spinach mixture in a large bowl. Add cottage cheese, shredded mozzarella, egg, salt, and pepper, and stir to mix thoroughly.

Spread 1 cup tomato mixture on the bottom of a slow cooker. Top with a complete covering of lasagna noodles. (Break noodles to fit all spaces, overlapping if necessary.) Spread ¾ cup of sauce over noodles. Top with ½ of the spinach mixture, spreading it evenly with the back of a spoon. Place another layer of lasagna noodles over spinach mixture. Spread ¾ cup of sauce atop noodles, then spread remaining spinach mixture over sauce. Add a final layer of noodles and top with remaining tomato sauce. Sprinkle Parmesan cheese over sauce.

Cook on low for 5 hours, until the noodles are tender and lasagna holds together when tested with a knife.

Tip: When assembling this incredibly easy lasagna, remember this little refrain: Sauce, noodles, sauce, spinach, noodles, sauce, spinach, noodles, sauce. Leftover lasagna freezes well. Thaw and reheat in the microwave before serving.