Spread ¼ cup pasta sauce around the bottom of a 4-quart crockpot.
Add a layer of uncooked lasagna noodles, breaking them if needed so that they will fit. Smear 1/2 cup ricotta cheese and 1/4 cup pesto onto the noodles.
Add a handful or two of baby spinach then top with a layer of mushrooms, Parmesan, and mozzarella.
Repeat layers (beginning with the sauce) until you've run out of ingredients.
Before closing the pot, put water into the empty pasta sauce jar, close the top, and shake. Pour this saucy water over the top of everything. Cook on high for about 4 hours.