Cook the onion in 1 tbsp olive oil until soft
Add the ground beef, and cook until no longer pink, breaking up with a spatula as you go. Drain grease if necessary
Season with sea salt, 1 tsp garlic powder and black pepper, and set aside to cool
In a medium bowl, mix the ricotta, 2 cups mozzarella, ½ cup Parmesan cheese, 1 tsp garlic powder and Italian seasoning until combined
Stir cooled meat mixture into the cheese mixture
In a large pot of boiling water, cook the manicotti noodles for 3-4 minutes or until they just start to soften. Drain with a slotted spoon, and remove to a large baking sheet sprayed with non stick cooking spray or olive oil to cool
Preheat oven to 350℉, and brush 1 tbsp olive oil in a large baking dish
Spoon about half of the marinara sauce on the bottom of the prepared baking dish
Stuff the manicotti with the filling mixture, and place in the baking dish in a single layer
Spoon remaining sauce on top of the manicotti, and sprinkle with 2 cups mozzarella and remaining Parmesan cheese
Bake for 30 minutes or until sauce is bubbling, pasta is heated through and cheese is melted