Lay tomato slices on a paper towel-lined baking sheet. Sprinkle both sides with salt and let sit for 10 minutes. This draws out excess moisture so they fry up crisp, not soggy. Pat completely dry with more paper towels.

In a shallow bowl, whisk together flour, cornmeal, black pepper, and cayenne if using. In another shallow bowl, whisk buttermilk and egg until smooth.

Working with one slice at a time, dredge in the cornmeal mixture, shake off excess, dip in buttermilk mixture, then dredge again in cornmeal mixture. Press gently so coating adheres. Place on a clean plate.

Heat 1/4 inch of oil in a large cast-iron skillet over medium-high heat until shimmering, about 350°F. Fry tomatoes in a single layer without crowding, 2-3 minutes per side, until deep golden and crispy. Adjust heat as needed to prevent burning.

Transfer to a wire rack set over a baking sheet. Sprinkle lightly with salt while hot. Let oil return to temperature between batches, adding more oil if needed.

Stack tomatoes on a platter, garnish with parsley, and serve hot with lemon wedges if desired. They are at their crispiest within 10 minutes of frying.