Preheat oven to 350 degrees. Begin to boil a pot of salted pasta water.
In a large, deep pan, add 2tbsp olive oil and 1tbsp butter and heat on medium-high heat. Add seasoned chicken and cook for about 2-3 minutes on each side. Remove from pan.
Lower heat to medium. In the same, deep pan chicken was cooked in, add 1tbsp of olive oil, onions, and shallots with a sprinkle of salt and cook until softened for about 2-3 minutes. Add minced garlic and cook another 30 seconds. Stir in chopped spinach with the juice of 1/2 a lemon and cook until wilted for 1-2 minutes. Remove from the pan, and squeeze out any excess juice from spinach.
In the same pan, add 1tbsp olive oil, 8 basil leaves, sliced garlic and Calabrian chili oil. Let it sizzle for a few minutes together, stirring and being careful to not allow garlic to burn.
Add can of tomatoes and season with 1tsp salt. Stir and reduce heat to low, cover and simmer for 10 minutes.
In the meantime, cook pasta shells 2 minutes less than what the package says (this is typically about 8 minutes). Drain and immediately rinse with cold water to stop the shells from cooking. Drizzle a little olive oil to prevent sticking and set aside in a single layer on a baking sheet.
In a large bowl, mix cooked chicken, ricotta cheese, 1.5 cups of mozzarella, 1 cup parmesan cheese, an egg, cooked spinach, basil and salt and pepper.
Stuff each shell with 2tbsp of chicken/ricotta filling. In a baking dish, spread a thin layer of tomato sauce on the bottom. Then arrange stuffed shells into the dish.
Pour remaining tomato sauce directly over the shells, then cover with remaining 1 cup of mozzarella cheese.
Cover with foil and bake for 20 minutes. Remove the foil and finish under the broiler for 1-2 minutes until golden and bubble. Enjoy!