Spray the bottom of a six-quart slow cooker with cooking spray. Place onions and garlic on bottom. Arrange chicken pieces over the onion and garlic.

In a food processor fitted with the knife attachment, blend the plums, soy sauce, wine, silan, olive oil, maple syrup, and salt and pepper. Pour over the chicken, taking care that all pieces are covered in sauce. Lay the whole scallion over the chicken. Scatter the chestnuts all around.

Cook on high for one hour and then on low for three hours, or cook on low for four to five hours.