1. Heat oven to 400ºF. Place the peas in a colander or large strainer and run under cold water until thawed; let drain.
2. In a medium bowl, combine the ricotta, peas, and 1/4 teaspoon each salt and pepper. Spoon half the marinara into an 8-inch square baking dish. Top with 2 layers of lasagna noodles (4 sheets).
3. Spread half the ricotta mixture over the noodles and top with a single layer of lasagna noodles (2 sheets). Spread the remaining ricotta mixture on the noodles. Top with 2 layers of noodles (4 sheets) and the remaining marinara.
4. Sprinkle the mozzarella on top. Loosely cover with foil and bake until the noodles are tender, 45 to 50 minutes.