melt butter in a large, wide frying pan over medium high heat

sprinkle chicken on both sides with cajun seasoning

add to pan and cook, uncovered until chicken is golden and cooked through, 4-5 minutes on each side

transfer chicken to a platter and cover loosely with foil

drain all but one tbsp of oil from the pan and return to heat

add tomatoes. using a wooden spoon scrape up and stir brown bits from pan

reduce heat to medium low

cover and simmer, stirring occasionally, until tomatoes break down and turn saucy, about five minutes

stir in onions for the last three minutes of cooking

serve sauce over chicken