Combine red pepper jelly, diced peach, mustard, and horseradish in a small saucepan and simmer for 5 minutes. Let cool to room temperature.
On a plate, stir together flour, salt and pepper, and cayenne pepper.
Whisk together buttermilk and eggs in a bowl.
Mix together cornmeal and panko crumbs on a second plate.
Pour oil to approximately ½-inch depth in a large skillet and heat over medium heat until oil is hot.
Dredge each tomato slice in flour, then dip in buttermilk/egg mixture, and then coat with cornmeal/panko mixture. Place about 5 slices in oil at a time (do not crowd) and cook until golden brown on both sides, about 3 minutes per side. Place on a paper towel-lined plate. Season with a little more salt while warm. Serve warm with pepper jelly sauce.