Bring a large pot of generously salted water to a boil. Add shells and cook until almost al dente.
Drain pasta, then place shells on a large baking sheet (so they won't stick together). Set aside.
While pasta is cooking, Add garlic and artichokes to the food processor. Pulse to chop.
Add spinach, ricotta, Parmesan, mozzarella, chicken and pesto to the artichoke mixture, salt and pepper to taste. Pulse until combined
While the pasta is cooking, Melt butter in a large skillet over medium heat, then stir in garlic and cook until fragrant (about 30 seconds), stirring constantly.
Sprinkle flour over butter and cook, stirring constantly, for about 2 minutes until combined and smooth.
Whisking constantly, slowly add milk. Bring mixture to a low boil, then reduce heat to a simmer.
Cook until thickened (about 10 minutes), stirring frequently.
Sauce should coat the back of a spoon when ready. Remove from heat, then add a dash of nutmeg, 2 tablespoons pesto and season generously with salt and pepper to taste.
Preheat oven to 350 degrees. Spray a 9 x 13-inch baking pan with nonstick spray, then coat the bottom with about ½ of the white sauce.
Fill shells with a generous spoonful of filling (I used a piping bag) and place in baking pan. (Crowd shells together to fit pan, if necessary.)
Pour remaining sauce over filled shells, then sprinkle with shredded mozzarella.
Bake for 30 minutes or until hot and stuffed shells lightly golden in spots, about 5 to 10 minutes more.
Serve shells hot, sprinkled with additional Parmesan cheese, if desired.