Cola Chicken Skewer Cups

6 boneless chicken breasts
1 can cola
3 garlic cloves, minced
1/4 cup balsamic vinegar
1/2 teaspoon cardamom
2 teaspoons kosher salt
1 teaspoon coarse black pepper
2 tablespoons ketchup
2 red bell peppers, cut into 1 inch x 2 inch strips
1 large onion, quartered and separated into strips

Cut chicken into 1 1/2 inch cubes. Combine all other ingredients except the onions and bell peppers, add to chicken, and marinate for 30 minutes. Remove chicken and thread onto skewers with peppers and onions. Reduce leftover marinade over high heat. Grill skewers on a grill pan, turning and basting with reduced marinade until the chicken is cooked through to at least 165°F.