First, liberally spray the inside of the slow cooker with cooking spray. Next, mix the pesto and ricotta cheese until well combined.
Now pour the tomato sauce to cover the bottom of the slow cooker (approximately 1/2 cup).
Next, top with enough broken lasagna noodles to cover the sauce.
Add 1/3 of the pesto and ricotta cheese mixture onto the noodles. There is no need to spread the mixture out with a spatula; it will do this naturally while it cooks.
Now add a handful of mozzarella and 1/4 of Parmesan cheese on top.
Repeat the above process 3 more times (tomato sauce, noodles, pesto and ricotta cheese mixture, mozzarella, and Parmesan cheese).
Now put the lid on. Cook on low for 5 hours or high for 3 hours.
When done, let it sit at room temperature for 20 to 30 minutes so the lasagna holds together when served.