Preheat oven to 350°F.
Boil a large pot of salted water. Add pasta. Cook for 8 to 10 minutes until al dente. Drain. Have a care not to over-cook. Shells that are too soft will be hard to stuff. Separate shells. Let stand on waxed paper or a non-stick surface.
Combine bread crumbs, chicken, egg, and onion, in large skillet over medium heat. Cook until chicken is white. Season with basil, oregano, pepper, and salt.
Stir in three-quarters of the shredded mozzarella and cheddar. Cook until cheeses melt completely.
Stuff shells with chicken mixture. I have found a soup spoon or a teaspoon a good way to stuff shells. Place stuffed shells in a 9x13 baking dish. Cover shells with tomato sauce. Top with remaining cheeses. Cover with foil and bake 45 minutes, or until lightly browned and bubbly. For a crispier crust on top, remove foil and place under broiler for 1-3 minutes.
When the shells are your main dish, serve with a side of broccoli or asparagus for a good flavor combination. A big, thick, slice of garlic bread is wonderful too!