Combine the onion, garlic, vinegar and soy sauce in the bottom of the slow cooker insert.
Dice the chicken into 1 to 1 1/2 inch chunks and toss in the marinade.
Toss to coat.
Cover and cook on low for 5 to 6 hours.
For a Thicker Sauce (right in the crockpot):
30 minute before serving turn the crock pot to high. Stir and tilt the lid to allow evaporation. The sauce will slightly thicken.
For a Quicker Thick Sauce:
Follow the directions above but ladle out at least half the liquid.
Heat over medium-high heat in a wide-bottomed saucepan or skillet. Bring to a boil then turn down to low. Stir every minute or two until the sauce thickens (about 10 minutes).
Freezer Meal Prep:
Prep the ingredients as directed and toss into a large freezer bag. Label and freeze for up to 6 weeks.