The gizzard and heart can yield some of the toughest bites you'll find in a chicken; both organs get a lot of work throughout the life of the bird and won't gracefully reach tenderness when fried without a little coaxing. Relieve them of their marked chewiness with the Southern treatment: a little pounding from the meat mallet and a cultured buttermilk bath.
Tenderizing
Rinse the gizzards and hearts under cool water and drain them on paper towels. Pat the gizzards and hearts dry with a paper towel and transfer a handful of them to a cutting board.
Cover the gizzards and hearts with a couple of sheets of plastic wrap and tuck the edges under the cutting board. Cover the plastic wrap with a thin cloth or kitchen towel.
Strike the gizzards and hearts several times with a meat mallet, the bottom of a saucepan, a rolling pin or whatever you have handy. You don't have to pound them to a uniform thickness, just strike them firmly a few times.
Transfer the gizzards and hearts to a bowl with a few aromatics, such as sliced onions and garlic. Cover them in buttermilk, and let them marinate in the refrigerator for 24 hours.
Frying
Drain the gizzards and hearts in a colander. Slice them into 1/4- to 1/2-inch pieces. Heat 3 to 4 inches of peanut or canola oil in a tall, wide, heavy-bottomed pot -- Dutch ovens and cast-iron pans work best -- on the stove with medium heat.
Mix a few cups of flour with salt and spices -- paprika is a classic -- and pour it in a dish. Whisk a couple of eggs in a bowl until smooth.
Check the oil temperature with an instant-read thermometer and work the heat as needed to keep it between 350 and 375 degrees Fahrenheit.
Dip the gizzards and hearts in the egg one piece at a time, and then coat it heavily with the flour mixture. Use one hand for dipping and the other for flouring. Gently drop the pieces in the oil; move them around for a few seconds using a slotted spoon to keep them from sticking.
Cook the gizzards and hearts until golden brown, about 3 to 4 minutes. Then transfer the pieces to a plate lined with paper towels, again using a slotted spoon. Garnish with fresh herbs and serve straight away.