Preheat oven to 400°F (200°C).

Bring a large pot of water to a boil. Add salt to taste, then cook 12 ounces jumbo pasta shells (about 20–24 shells) until al dente according to package instructions. Drain shells and spread them out on a baking sheet or plate to cool slightly so they are easy to handle.

While the shells cook, heat a large skillet over medium-high heat. Add 1 pound ground beef, 1/2 cup diced onion, and 4 cloves minced garlic. Cook 3–4 minutes, breaking up the beef, until the beef is browned and the onion is softened. If excess fat accumulates, drain it from the pan.

Add 2 cups loosely packed spinach to the skillet and cook about 2 minutes, stirring, until the spinach is wilted.

Stir in 1 teaspoon basil, 1 teaspoon garlic powder, 1 teaspoon oregano, and salt and pepper to taste. Then stir in 1 cup of the 2 cups marinara sauce. Remove the skillet from heat.

Arrange the cooked shells in a 13x9-inch baking dish (or a similar oven-safe pan), open side up.

Fill each shell with about 2–3 tablespoons of the meat and spinach mixture. Use a spoon to mound the filling into each shell.

Pour the remaining 1 cup marinara sauce evenly over the filled shells.

Sprinkle 1 cup shredded mozzarella cheese and 1/2 cup shredded Parmesan cheese evenly over the top. Cover the baking dish with foil.

Bake covered for 15 minutes at 400°F. Remove the foil and broil 1–2 minutes, watching carefully, until the cheese is lightly browned if desired.

Remove from oven and let rest a few minutes before serving.