For the tomatoes: In a large cast-iron skillet, heat 1/2-inch of peanut oil to 350 to 375 degrees F over medium-high heat. It is best to use a fry thermometer to maintain and monitor the correct temperature. Add a few tablespoons of the reserved bacon grease to the oil if you'd like.
Slice the tomatoes into 1/4-inch-thick slices. In a shallow bowl, whisk together the egg and buttermilk. Add a few dashes of hot sauce to the egg mixture, if desired. In another shallow bowl combine the flour, cornmeal, cornstarch and Old Bay seasoning. Add a generous pinch of salt and a few grinds of black pepper.
One at a time, dip a tomato slice into the egg mixture, and then dredge the slice in the cornmeal mixture. Make sure to generously coat all sides. Repeat with a few more slices. Using long tongs, add a couple of dredged tomatoes to the fry pan (depending on the size of the pan) and allow to cook until golden brown, about 1 to 2 minutes. Carefully flip one time if necessary. Remove the tomatoes to a sheet pan lined with a large brown paper bag, which will absorb grease without making the tomatoes soggy. Sprinkle the hot tomatoes with kosher salt. Continue frying the additional slices; assemble sandwiches immediately.
For the mayonnaise (which can be made up to two days in advance): In a medium-sized bowl, whisk together the egg yolk, lemon juice, Dijon mustard, cider vinegar and a pinch of salt. Using a hand-held mixer on medium speed, slowly add the canola oil drop by drop until the mixture begins to thicken and lighten (this can take a couple of minutes). After adding about 1/4 cup of the oil, the mixture should be stable and emulsified. You can then add the oil in a slow, steady stream until all of the oil is incorporated. Taste for seasoning. Add salt if necessary and quite a few generous turns of the pepper mill set on the coarsest setting.
For the sandwiches: Slather one slice of bread with the black peppercorn mayonnaise. On the other slice, arrange a layer of lettuce, followed by a couple of fried green tomatoes (or one, if the tomato is very large), and by pieces of bacon (broken in half if necessary). Top the sandwich with the other slice of bread and enjoy immediately.