Wilted Spinach Salad with Chicken and Mandarin Oranges

boneless, skinless chicken breast halves, about 6 oz. each, pounded 1/2-inch thick

Salt and pepper

unsalted butter

vegetable oil

small red onion, sliced

red wine vinegar

baby spinach (about 5 oz.)

mandarin orange sections, drained

dry roasted peanuts, chopped