Preheat your oven to 425°F (218°C).
Slice the eggplant into ¼-inch thick rounds. Lightly salt the slices and let them sit for 10 minutes to draw out moisture, then pat dry.
In a shallow bowl, whisk the egg. In another bowl, place the whole wheat breadcrumbs.
Dip each eggplant slice first into the beaten egg, then coat with breadcrumbs, pressing gently to adhere.
Arrange the breaded eggplant slices on a baking tray lined with parchment paper. Lightly drizzle or spray with olive oil.
Bake the eggplant for 20-25 minutes, flipping halfway through, until crispy and golden.
While the eggplant bakes, gently heat the marinara sauce in a small saucepan.
Once baked, place the eggplant slices on the bottom half of the toasted whole wheat bun. Spoon a layer of marinara sauce over the eggplant and top with the low-fat mozzarella cheese.
Return the assembled sandwich to the oven for 3-5 minutes to melt the cheese.
Remove from the oven, garnish with fresh basil leaves, assemble the top bun, and serve immediately.