In a large skillet on medium-high heat, heat olive oil until simmering. Add the chicken breasts and sauté for 4 minutes on each side. For the the best sear, do not flip the chicken more than once.
Remove chicken breasts to a plate or platter, tenting loosely with foil. Add onion and garlic to skillet and cook 1 minute. Add lemon juice, lemon zest, and chicken broth, stirring until smooth. Simmer until bubbly.
Remove skillet from heat and add butter and capers. Using tongs, dip each chicken breast into the sauce, coating both sides. Serve with slices of lemon as garnish (optional).
Leftovers can be stored in an airtight container in the fridge for up to 3 days.