Step 1: Preheat the Oven. Begin by preheating your oven to 375°F (190°C).
Step 2: Prepare the Cheese Mixture. In a large mixing bowl, combine the ricotta cheese, shredded Parmesan cheese, salt, pepper, and egg. Mix until creamy, then fold in the thawed spinach.
Step 3: Assemble the Lasagna. Spread 1 cup of pasta sauce on the bottom of the baking dish. Lay 4 noodles, half the ricotta mixture, pesto, 1 cup of sauce, and 1 cup of fontina cheese.
Step 4: Repeat the Layers. Add another 4 noodles, the remaining ricotta mixture, pesto, and 1 cup of sauce. Finish with the last 4 noodles, remaining sauce, and fontina cheese.
Step 5: Bake. Cover with foil and bake for 25 minutes. Remove foil and bake for an additional 15-20 minutes until bubbly and golden brown.
Step 6: Let it Rest. Allow to rest for 10-15 minutes before slicing and serving.