Lightly spray the inside of a slow cooker crock with nonstick cooking spray. Spread 1/2 cup marinara sauce on the bottom of the slow cooker, then add a layer of noodles (you may have to cut or break noodles to fit in a single layer).
Cover the noodles with 3/4 cup ricotta cheese, 1/4 cup green peas, 1 cup baby spinach leaves, 1 cup sliced zucchini and 3/4 cup marinara sauce. Repeat with two additional layers of noodles, ricotta, vegetables and sauce; you will have three layers in all. End with a final layer of noodles, the remaining marinara sauce and the mozzarella cheese.
Cover with the lid and cook on high for 3 hours or 5 to 6 hours on low. Turn the slow cooker off and let the lasagna sit for at least 30 minutes before serving (this allows it to firm up).
Garnish with parsley and serve.