Preheat your oven to 350°F (175°C).

Cook the jumbo shells in boiling salted water until al dente (about 10-12 minutes). Don't overcook! Drain, rinse briefly with cool water, and set aside.

Heat cooking oil in a large skillet over medium heat. Add ground beef and cook until browned. Drain excess fat.

Add diced onion and minced garlic to the skillet with the beef. Sauté until onion is translucent and garlic is fragrant (about 3-5 minutes).

Stir in the marinara sauce, tomato paste, Italian seasoning, salt, and garlic powder. Simmer for 3-5 minutes.

Carefully stuff each cooked shell with the beef and sauce mixture. Arrange the stuffed shells in a 9x13 inch baking dish.

Pour any remaining sauce over the stuffed shells.

Bake uncovered for 20-25 minutes, or until heated through and bubbly.