Preheat oven to 350°F and set aside a 3-quart casserole dish.
Cook pasta in salted water just to al dente, then drain and spread on a rimmed baking dish to cool.
Meanwhile, heat olive oil in a sauté pan over medium-high heat. Cook chicken until no pink remains, then stir in garlic, salt, pepper, oregano, rosemary, thyme, and crushed red pepper and cook 2 minutes or until fragrant. Remove from heat and let cool slightly.
In a large bowl, combine ricotta, 1 cup mozzarella, parmesan, and egg. Stir in cooked chicken.
Spread 1 cup of marinara sauce onto bottom of casserole dish. Stuff shells using a tablespoon and arrange onto dish. Repeat until all the dish is full or you run out of filling or shells. Pour remaining sauce over the top evenly.
Bake for 30 minutes, then sprinkle on remaining 1 cup mozzarella and bake for 10 minutes more, or until cheese is melted and dish is bubbly.
Chicken and cheese mixture can both be made in advance to save time at dinner. Store separately in the refrigerator and mix together when ready to stuff and bake.