In a slow-cooker, stir together the mustard, onion, garlic, olive oil, apple cider vinegar, salt, pepper, and paprika. Then stir in the chicken.
2/3 cup yellow mustard, 1/2 onion, 2 tablespoons olive oil, 2 cloves garlic, 1 tablespoon salt, 2 teaspoons paprika , 1 teaspoon apple cider vinegar, 1/2 teaspoon pepper, 4 large chicken breasts