Heat butter and olive oil in pan over medium to medium high heat.
Mix flour, salt and pepper on a plate or deep dish- set aside.
Pound chicken breast to flatten. Dredge chicken in flour mixture.
Brown chicken in olive oil and butter. Once brown (about 5 mins on each side) remove chicken and keep covered.
Add chopped onion to pan and brown- it doesn't have to be completely tender- about 3-4 minutes
Add Marsala wine, horseradish and capers. Stir to combine.
If you don't have a pan that is oven safe pour pan ingredients in oven safe dish. Place chicken on top of sauce and cover. Cook about 30 minute I made up some pasta and put the sauce over the pasta, you could also use rice.