large chicken breasts, skinless and boneless (about 1 1/2 lbs.), halved horizontally, and pounded flat • Kosher salt and black pepper • all-purpose flour, for drenching • unsalted butter • Extra-virgin olive oil • shallots, peeled and sliced lengthwise • lemon, halved (half thinly sliced and seeds removed; half juiced, about 2 tablespoons) • chicken stock • capers, drained • Fresh parsley, coarsely chopped, for garnish