4 boneless, skinless chicken breast halves each about 8oz, pounded to ½ inch thickness
½ cup spicy beer marinade
3 large yellow onions, cut into 3/16 inch rounds
¼ cup olive oil
Salt and freshly ground pepper, to taste

Put the chicken breasts in a large sealable plastic bag, add the marinade and seal the bag. Refrigerate for at least 8 hours or up to overnight.
Preheat a stovetop grill pan over medium-high heat. In a bowl, stir together the onions, olive oil, salt and pepper. Working in batches, place the onions in a single layer on the pan and cook, turning once, until tender and nicely grill-marked, 8 to 10 minutes per batch. Transfer the onions to a bowl and cover with aluminum foil.
Wipe out the grill pan. Remove the chicken from the marinade and pat the chicken dry with paper towels. Arrange the chicken on the pan and cook, turning once, until the chicken is nicely grill-marked and cooked through, 12 to 14 minutes total.
Transfer the chicken to a cutting board, cover loosely with aluminum foil and let rest for 5 minutes. Cut the chicken against the grain into slices ¼ inch thick and serve with a mound of the grilled onions.