Heat your oven to 375°F and grease your baking dish with spray or olive oil.
Boil a big pot of salty water and cook penne pasta for 8-10 minutes until tender.
Drain the pasta in a colander and set it aside while you get everything else ready.
Heat olive oil in a pan and cook fresh spinach for 3-4 minutes until it wilts down.
Press the cooked spinach hard with a spoon or paper towels to get the water out.
Mix ricotta cheese, egg, Parmesan, salt, pepper, and garlic powder in a big bowl until smooth.
Add the pasta, spinach, and marinara sauce to the ricotta and stir really well.
Pour everything into your greased dish and spread it out so it's nice and even.
Top with mozzarella cheese and the rest of the Parmesan for a golden cheesy crust.
Bake for 25-30 minutes until the cheese melts and turns light golden brown on top.
Take it out and let it sit for 5-10 minutes before you cut and serve it.