Fill a tea kettle or 2 quart saucepan with water and bring to a boil. Remove excess fat from chilled chicken and place in colander over a large bowl. Spread out with a fork. Pour hot water over meat through colander.
Place chicken in plastic container with tight fitting lid.
Add onions, chili powder, oregano, garlic powder, cumin, and paprika to chicken.
Refrigerate chicken overnight in plastic container with tight fitting lid.
To make tacos, place chicken mixture in a pan and heat slowly or heat in microwave for 2–3 minutes, stirring after 1½ minutes to heat evenly. Combine finely shredded lettuce and cabbage. Mix cheeses together. Place ¼ cup heated chicken mixture in a tortilla and top with cheese and vegetables.