Whisk the egg and buttermilk together in a bowl and set aside. *If you are wondering what to do with the rest of the container of buttermilk, make pancakes tomorrow! Or biscuits. Or a really rad salad dressing. Remind me to give you guys these recipes later, or I'll forget.
Put ¼ flour in a shallow pan with half the seasoning salt. In another shallow pan mix the cornmeal and the rest of the flour. Yes, there will be a ton of dishes. It's worth it.
Dip the sliced tomatoes in the flour/salt, then egg/milk. then flour/cornmeal. Drop the tomatoes in carefully, one at a time when the oil gets to be 350' (shimmery but not smokey) in one layer. Cook for 2-3 minutes per side, then grab out with your tongs and let drain and cool down for a minute.
Sprinkle some more seasoning salt to taste. It will probably take you 2 or 3 batches to finish all of them. Build the sandwich.