Put tomato slices in a colander to drain excess liquid. Discard end slices of tomatoes.
Whisk milk and egg together in a bowl. Stir flour, cornmeal, panko, salt, onion powder, garlic powder, black pepper, and cayenne pepper together in a separate bowl.
Gently press a tomato slice into the flour mixture to coat and shake to remove excess flour. Dip into the beaten egg, then press again into the flour mixture. Keep breaded tomatoes on a plate while breading the rest; do not stack.
Pour enough olive oil into a large skillet to reach about 1/2 inch deep. Place skillet over medium heat and heat oil until hot but not smoking. Cook 4 to 5 tomato slices at a time in the hot oil until browned on the bottom, 2 to 3 minutes; turn tomatoes and continue cooking until browned on the other side, 2 to 3 minutes more. Reheat oil between batches, adding more oil as needed to maintain 1/2-inch depth.