Bring a large pot of salted water to a boil. Cook the jumbo shells 2 minutes less than package directions until just flexible but still quite firm - they will finish cooking in the oven. Drain and rinse under cool water. Lay shells in a single layer on a baking sheet so they don't stick together.

Heat a large skillet over medium-high heat. Add the ground beef and cook, breaking it up with a wooden spoon, until no pink remains, about 6-8 minutes. Drain excess fat. Stir in half the minced garlic, 1/2 teaspoon Italian seasoning, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Cook 1 minute more until fragrant. Transfer to a plate and let cool slightly.

In a large bowl, combine ricotta, beaten egg, remaining garlic, 1 teaspoon Italian seasoning, 1/2 teaspoon salt, and 1/4 teaspoon pepper. Fold in the cooled ground beef until evenly distributed.

Spread 1 cup of marinara sauce across the bottom of a 9x13 inch baking dish. Fill each shell with about 2 tablespoons of the beef-ricotta mixture, packing it in gently. Arrange stuffed shells seam-side up in the dish, nestling them close together. You should fill about 18-20 shells.

Pour remaining marinara sauce evenly over and around the shells. Sprinkle mozzarella and Parmesan over the top, covering completely.

Cover tightly with foil and bake at 375°F for 20 minutes. Remove foil and bake 10-15 minutes more until cheese is melted and spotty golden brown. Let rest 10 minutes before serving. Scatter fresh basil over the top.