Do this step if you think your eggplant is particularly bitter (it starts to ooze gray liquid shortly after slicing). If not skip to step two. Place sliced eggplant in a colander covering both sides with kosher salt between each layer. Place a bowl under colander or in the sink to collect the expelled liquid. Cover eggplant with a plate slightly smaller than the colander, and then place a heavy pot on top to weight it down for 30 minutes to 2 hours (depending on how bitter it is). Rinse under cold water and pat dry.
Make tomato sauce. Place 2 tbsp of olive oil, onions, and a pinch of salt in a small pot. Let onions sweat on low heat for about 10 minutes. Mix in tomatoes and garlic and simmer for another 10 - 15 minutes, stirring occasionally. Turn off heat and mix in fresh basil. Check for salt and pepper.
Meanwhile heat grill or grill pan. Brush with oil. Grill eggplant for a few minutes per side. Add salt and pepper to taste.
Make 2 stacks layering about 4 eggplant slices per stack (depending on your eggplant), placing a bit of tomato sauce and a slice of mozzarella cheese between each layer. And ending with a bit of tomato sauce and a slice of mozzarella cheese. Serve warm. Enjoy!