Put the chicken into a 6- to 8-quart slow cooker, in a single layer. Add the oil and the spices, plus 1 tablespoon salt and several generous grinds of black pepper. Mix well with a spatula to evenly coat all sides of the chicken with the oil and spices.
Cook on low until the thickest part of the chicken breast reaches 155 degrees, 2 to 2½ hours. (If necessary, it’s fine to leave the chicken on the warm setting for an hour or two.)
Remove the breasts from the slow cooker and let them rest for 5 minutes before slicing and serving, spritzed with lemon juice. Or store in the refrigerator for up to 4 days or in the freezer for up to 3 months.
