Preheat your oven to 375°F (190°C).
Slice the eggplant into 1/4-inch thick rounds. Lightly season with salt and set aside for 10 minutes to draw excess moisture, then pat dry.
Finely dice the small onion and mince the garlic.
In a nonstick skillet, heat a splash of water or cooking spray over medium heat and add the diced onion and garlic. Sauté until softened, about 3-4 minutes.
Add the lean ground beef to the skillet, breaking it up as it cooks. Season with salt, black pepper, dried oregano, and basil. Cook until the beef is browned and cooked through, about 6-8 minutes. If desired, stir in the low-sodium beef broth to keep the mixture moist, then remove from heat.
In a small saucepan over medium heat, melt the unsalted butter. Stir in the whole wheat flour and cook for 1 minute, creating a roux. Gradually whisk in the skim milk (and a bit of the beef broth if you prefer a thinner consistency) until the sauce thickens. Season lightly with salt and pepper to taste to create your bechamel sauce.
In a baking dish, layer a portion of the eggplant slices at the bottom. Spoon half of the beef mixture over the eggplant, then drizzle a layer of the creamy bechamel sauce. Repeat the layers with the remaining eggplant, beef mixture, and finish off with a final drizzle of bechamel sauce on top.
Bake in the preheated oven for 20-25 minutes until the casserole is heated through and the flavors meld together.
Let stand for a few minutes before serving. Enjoy your layered eggplant and beef casserole as a hearty, comforting meal.