Combine all ingredients and pour mixture over chicken.
Halve each chicken fillet, lengthwise. Cut each half in 3 pieces, season with salt and pepper, and place them in a bowl with marinade. Cover with plastic film and refrigerate for 2 – 3 hours.
Thread 1 chicken piece onto skewer; continue with a piece of onion, chicken, red and green pepper, chicken, mushroom and finish with a piece of chicken. Repeat with the remaining skewers.
Place skewers over hot grill and brush remaining marinade several times. Turn skewers 1 or 2 times until tender and browned.