Preheat oven to 375°.
Cook shells according to package directions and let cool enough to handle.
In a large bowl, combine the chicken, spinach, ricotta, egg, Italian seasoning, oregano and 1 cup of shredded mozzarella cheese.
Once the shells are cool enough to handle, stuff the shells with a heaping tablespoon of the chicken filling and place the shells, filling side up, in a 9x13 dish. 6 shells across and 6 shells down.
Melt the butter in a skillet over medium-low heat, then add the garlic and saute until the garlic is really fragrant, about 1-2 minutes.
Sprinkle in the flour and whisk it together, let the flour cook 2-3 minutes.
Slowly pour in the cream while whisking the entire time. Bring the sauce to a low boil and then reduce the heat to low and let simmer until thickened, about 2-3 minutes.
Whisk in the Parmesan cheese until completely melted. Remove sauce from heat and evenly pour the sauce over the shells.
Top the shells with the remaining 2 cups of shredded mozzarella cheese.
Bake for 30 minutes until bubbly and the cheese is completely melted and starting to brown.