Preheat oven to 375 degrees F (190 degrees C). Grease a 9x13 inch baking dish.
Season chicken with salt and pepper. In a large skillet over medium heat, heat olive oil.
Add chicken to the skillet and cook until browned. Remove from heat and set aside.
In the same skillet, sauté onion and garlic until softened.
Return chicken to the skillet and add cream of chicken soup and milk. Stir until combined.
Set aside 1/2 cup of the mixture for later use.
Pour the remaining mixture into the prepared baking dish.
Cook pasta shells according to package instructions. Drain and add to the baking dish.
Stir in cheddar cheese and Parmesan cheese.
Bake for 25 minutes, or until bubbly."