Cook pasta shells al dente, drain and set aside. *Tip: I like to cook 2-3 extra shells just in case some get damaged as they cook. (So you can cook 25, just in case.)

Cook chicken in a pan, over medium heat, seasoned with some salt and pepper. Once chicken is done, take it out and let it cool enough to be handled. Once it's cooled, dice the chicken.

Preheat oven to 375. Lightly grease a 9x13 casserole dish.

In one mixing bowl, combine diced, cooked chicken, olive oil, Italian cheese mix and pesto. Mix very well and set side.

In another mixing bowl, combine all ingredients for ricotta mixture. Mix until all smooth and set aside.

In a sauce pot, melt butter over medium heat. Whisk in flour until combined. Start adding milk slowly while still whisking so that the mixture is smooth. Add salt, pepper and dry Parmesan cheese. Bring sauce to simmer, stirring often.

Pour sauce all over the bottom of the prepared casserole dish.

Stuff each shell with some ricotta mixture, filling them about 1/3 full, and then some pesto chicken mixture.

Nestle stuffed shells in the casserole dish, in the sauce, one right next to the other.

Spread mozzarella cheese over the top of stuffed shells.

Bake for about 20 minutes.

*You can grate some Parmesan cheese right over the cooked shells as you serve it.