Method

On cutting board and using kitchen shears, hold 1 of the chickens breast side down; cut off wing tips at first joint. Cut along both sides of backbone. Turn over; flatten with heels of hands. Turn chicken breast side down. With chef's knife, cut down centre of breast to divide in half. Cut off each leg; cut into thigh and drumstick pieces. Cut off wings; cut breast diagonally into 2 pieces. Trim all excess fat and loose skin. Repeat with remaining chicken.

Place chicken in large bowl; add onions, thyme sprigs and buttermilk, turning chicken to coat. Cover and marinate in refrigerator for 2 hours. (Make-ahead: Refrigerate for up to 24 hours.)

In large bowl, whisk flour, thyme, salt, cayenne and black pepper. Discarding marinade, press chicken into flour mixture, turning to coat. Keeping pieces separate, place on waxed paper-lined rimmed baking sheet; refrigerate until coating is set, at least 15 minutes or up to 1 hour.

Heat two 10- or 12-inch (25 or 30 cm) cast-iron skillets over medium heat. Divide 2 cups (500 mL) of the shortening evenly between skillets; heat until speck of flour mixture sizzles and browns. Place half of the chicken, fleshiest side down and in single layer, in skillets. Cover and fry, turning halfway through, until crisp, brown and juices run clear when chicken is pierced, about 18 minutes. Drain on paper towel-lined platter.

Strain melted shortening to remove any brown bits. If necessary, add enough of the remaining shortening to make 1 cup (250 mL) in each skillet; fry remaining chicken.