Heat 1 teaspoon olive oil in a large nonstick skillet. Add onions and garlic; stir and cook for 1 minute.
Add sausage to the skillet and continue to cook and stir occasionally for about 8 to 10 minutes, or until sausage is browned; drain, then stir in the diced tomatoes. Set aside.
Heat remaining olive oil in a nonstick skillet.
Pat dry the eggplant slices, add them to the skillet, and cook just until lightly browned on both sides, about 1 to 2 minutes.
Assemble The Casserole
Spread 1 cup of meat sauce on the bottom of a 9x13-inch baking dish.
Arrange about 6 slices of eggplant over the meat sauce.
Add more meat sauce over the eggplants.
Spread 1/3 of the bechamel sauce.
Repeat with 2 more layers, and top with remaining bechamel sauce.
Sprinkle the top with cheese.
Bake for 25 to 30 minutes, or until bubbly and top is lightly browned.
Remove from oven and let stand 10 to 15 minutes.
Serve.
Notes
Prepping the Eggplant: Salting the eggplant slices and letting them rest draws out the excess moisture and helps to remove any bitterness. Just remember to pat them dry before cooking.
Uniform Slicing: Using a mandolin to slice the eggplant ensures uniform thickness, allowing the slices to cook evenly. You will need about 18 to 20 thin slices of eggplant.
Swap Bechamel: Use a jar of Alfredo sauce if bechamel isn't an option.
Let it Rest: Let the casserole rest after baking for the best texture.
Try Variations: Add veggies like zucchini or peppers, or replace the sausage with a veggie option.