Stuffed Chicken Breast with Prosciutto, Pears and Brie

large boneless, skinless chicken breasts (16 oz total)

Kosher salt and pepper,

olive oil

chopped onion

slices prosciutto, chopped

sage leaves, chopped

medium pear, peeled and diced into 1/2-inch pieces

Brie, skin removed, divided

baby arugula

cooking spray

cooking twine