Heat oil in a skillet over medium-high heat. Brown chicken in hot oil, about 2 1/2 minutes per side.
While chicken is cooking, mix brown sugar, soy sauce, honey, garlic, and ginger together in a small bowl.
Transfer chicken to the bottom of a slow cooker. Place pineapple and onion on top of the chicken. Pour sauce over top.
Cover and cook on High for 3 hours.
Preheat the oven to 500 degrees F (260 degrees C).
Remove chicken from the slow cooker and place on a baking sheet.
Bake chicken in the preheated oven until no longer pink at the bone and the juices run clear, about 15 minutes. An instant-read thermometer inserted near the bone should read 165 degrees F (74 degrees C).
Meanwhile, mix cornstarch and water together in a small bowl. Add it to the slow cooker, stir, and cook until sauce has thickened, about 30 minutes. Add snap peas and chicken and cook until heated through, 15 to 20 minutes more.