Place chicken between 2 sheets of plastic wrap and pound lightly with a meat mallet or rolling pin to an even thickness of about 1/2 In. (may skip to save time).
Coarsely chop the onion.
Coarsely chop the capers.
In a large skillet, warm the oil over medium-high heat until hot but not smoking.
Add the chicken and brown all over, about 5 min. per side.
Remove chicken to a plate and cover loosely with foil to keep warm.
Add butter to skillet and heat until melted.
Add the onion and cook until it begins to brown.
Stir in the flour and cook, stirring, until the flour is no longer visible.
Stir in the chicken broth, lemon juice, lemon zest (if using),capers and pepper, and bring to boil over medium-high heat.
Return the chicken and any juices that have accumulated on the plate to the skillet.
Reduce the heat to low. Cover and simmer until the chicken is cooked through.