In a pan, heat the oil and brown the chicken over high heat. Add the onions and garlic, and cook for a few minutes. Add the remaining ingredients and stir. Set the heat to low and continue to cook, covered, for approximately 30 minutes, or until chicken comes apart with a fork.
For the pickled onions
In a saucepan, boil the vinegar with the sugar, then let cool. Place the red onions in a Mason jar, pour the vinegar mixture over top, and close the lid. Pickled onions keep at leat one month in the fridge.
Shred the chicken and serve with the tacos and toppings.