Preheat oven to 375°F (190°C).
In a saucepan, heat olive oil over medium heat. Add minced garlic and cook until fragrant.
Add marinara sauce to the saucepan, season with salt and pepper, and simmer for 10 minutes.
In a mixing bowl, combine ricotta cheese, thawed spinach, and half of the grated Parmesan cheese.
Spread a thin layer of marinara sauce on the bottom of a baking dish.
Layer 3 lasagna noodles over the sauce. Spread ⅓ of the ricotta mixture over the noodles, followed by ⅓ of the mozzarella cheese and ⅓ of the remaining marinara sauce. Repeat layers twice more.
Top with remaining Parmesan cheese.
Cover with aluminum foil and bake for 25 minutes. Remove foil and bake for an additional 20 minutes, or until cheese is bubbly and golden brown.
Let lasagna cool for 10 minutes before serving.