According to Dr. Oz,in each of the 4 pairs of foods, each pair enhances cancer prevention/fighting abilities3x than either food alone in the pair:
Chicken & Broccoli (Selenium & Sulforaphane)
Avocado and Salsa (Healthy Fat and Lycopene)
Yellow Onions and Tumeric (Quercetin and Curcumin)
Watercress and Salmon (Watercress enhances Omega-3's)
Quercetin can also be found in whole apples (with skins).