If your eggplant is quite big and perhaps not as fresh as you would like, it may require 'salting' to remove some of its bitter flavour. Pour 1 tsp salt over eggplant slices and allow to drain in a colander in the sink for about 30 minutes. Rinse well, pat dry and continue on with the following recipe steps.
Preheat oven to 425F. Place flour in a medium bowl. Place egg and water in a 2nd bowl and whisk until slightly frothy. Place breadcrumbs, ¼ tsp salt and ⅛ tsp pepper in a 3rd bowl and mix until combined.
Line a baking sheet with aluminum foil. Brush 2 Tbsp canola oil evenly over foil lined sheet. One slice at a time, dredge eggplant in flour bowl, followed by the egg bowl being sure to coat both sides, and finally the breadcrumb bowl coating both sides equally. Transfer to baking sheet. Bake eggplant for 25 minutes, flipping the slices halfway through, until breadcrumbs are golden brown.
Meanwhile, heat remaining canola oil in a frying pan over medium high heat. Sauté onions, peppers and remaining salt and pepper until very soft, 8-10 minutes.
Build each sandwich with 2-3 slices of eggplant, ¼ of the pepper and onion mixture, 2 Tbsp parmesan cheese and tomato sauce (I will leave the amount of sauce to your discretion).