Crock Pot Turkey Spinach Lasagna

My mom found this and raves about it, and everyone she's made it for raves about it, so it's on our menu for this month!

Ingredients:
1/2 pound ground turkey
1 large onion
1 TBSP olive oil
1 26 oz jar marinara sauce
1 14 oz can diced tomatoes
1 9 oz package oven ready lasagna noodles
2 cups part skim ricotta cheese
1 8 oz package grated part skim mozzarella cheese
1 10 oz package frozen chopped spinach, thawed and squeezed dry
4 TBSP grated parmesan cheese
1/2 tsp salt
1 TBSP oregano

Directions:
1.  In a large skillet over medium heat, saute turkey in oil until it is no longer pink and then drain.
2.  In a large bowl combine turkey, marinara sauce, and tomatoes.
3.  Coat a large slow cooker with cooking spray.  Pour 1 cup of the turkey tomato mixture into the cooker.  Layer noodles, ricotta, mozzarella, spinach and sauce.  Repeat layers twice, ending with the turkey-tomato sauce.  Use all the spinach in the first two layers, ending with sauce and then cheese.
4.  Cover and cook for 2 1/2 hours on low or until noodles are soft and cheese is melted.  Sprinkle with grated parmesan.