Place eggplant slices in a strainer and sprinkle with a little kosher salt. Allow to sit for 30 minutes to release any bitter juices. Squeeze out liquid and lay on a paper towel to dry. Place beaten egg in one plate and potato starch in another. Dip each eggplant slice into the egg and then into the potato starch.

Heat enough oil to coat the bottom of a large frying pan. Fry each eggplant slice until golden (approximately 3 minutes), then flip over and fry on the second side. Remove eggplant slices from frying pan and set aside.

Heat oil in a frying pan and sauté onion for 10 minutes, stirring occasionally so onions do not  burn. Remove from heat and place in a mixing bowl. Add frozen spinach, egg, salt, garlic powder, and pepper. Mix well.

Mix ingredients together in a large container and divide in half.

Preheat oven to 350 degrees Fahrenheit (180 degrees Celsius). 

Spread one to two tablespoons of the spinach mixture on each eggplant slice and roll up.

Arrange the eggplant rolls in a nine-inch square baking dish that fits the rolls tightly. Pour half of the tomato sauce mixture over the eggplant roll-ups and cover.

Bake for 25–30 minutes. Uncover and bake another 10 minutes. Before serving, heat the rest of the tomato sauce in a small saucepan and pour over eggplant roll-ups. Serve warm.