Use a 4 or a 6 quart slow cooker.
Put a spoonful of pasta sauce (about ¼ cup) into the bottom of your cooker and swirl it around.
Add a layer of uncooked lasagna noodles (break them to make a layer). Smear ricotta cheese and pesto onto the noodles.
Add a handful or two of baby spinach, and top with a layer of Parmesan and sliced mozzarella cheeses.
Repeat layers until you've run out of ingredients, squishing down the spinach to make it all fit.
Before closing the pot, put ¼ cup of water into the empty pasta sauce jar, close and shake, then pour this saucy water over the top.
Cover and cook on low for 6 hours, or on high for about 3 to 4 hours.
It's done when the top layer begins to brown and the cheese is melted and bubbly.
Taste-test a noodle to check texture.
Uncover, and let it sit for 10 to 15 minutes before serving.