Heat the oil in a large skillet over medium-high heat. Working in batches, brown the chicken skin-side down for 4-5 minutes until golden. Transfer to the slow cooker.
Pour off all but about a tablespoon of the fat. Add the smashed garlic to the skillet and cook for 30-45 seconds until fragrant, then scrape it over the chicken.
Add the vinegar, soy sauce, water, bay leaves, crushed peppercorns, and brown sugar to the slow cooker. Do not stir - adobo is traditionally left alone until the vinegar has cooked off its raw edge.
Cover and cook on LOW for 5 hours, or HIGH for 2 1/2 to 3 hours, until the chicken is tender and pulling from the bone.
Transfer the chicken pieces to a foil-lined sheet pan, skin-side up.
Skim the fat from the sauce. Simmer it in a saucepan for 8-10 minutes to thicken, or stir the cornstarch slurry into the slow cooker and cook on HIGH for 15 minutes.
Brush the chicken with a few spoonfuls of sauce and broil 6 inches from the element for 4-6 minutes, until the skin blisters and browns. Watch closely - the sugar can burn quickly.
Serve over rice with plenty of sauce spooned over, topped with green onions, alongside something crunchy and cool.