Whisk the buttermilk and eggs together in a bowl. Place the flour in a second bowl. In a third bowl, combine the breadcrumbs, cornstarch, and salt.
Gently dip each slice of tomato into the flour, then the egg mixture. Repeat this process once more, then dip the tomato into the breadcrumb mixture to coat both sides.
Heat the oil to 375°F in a heavy-bottomed pot. Gently add the coated tomato slices to the oil and fry on each side until golden brown. Place onto paper towels or a metal rack to drain.
Notes
Heat the oil to 375°F to crisp the crust without overcooking the tomatoes.
Give each tomato slice ample room in the pot so they cook evenly and don't stick together.
Drain well after frying to maintain the coating's crispness.