If using the optional eggplant layer, they need to be roasted first in a 400 degree oven.
Slice eggplants lengthwise about ¾ inch thick.
Brush both sides of slices with oil and season with salt and pepper.
Roast until soft and golden, about 25 minutes, turning each slice halfway through cooking time.
Cook sausage or beef in skillet with garlic until browned then drain off any grease.
Add canned tomatoes, sauce, tomato paste and sauce.
Mix until meat and sauce are well combined. In medium bowl, beat eggs then stir in ricotta and parmesan cheese.
Grease a rectangular 9×13 baking dish with the olive oil, add a large dollop of meat sauce and spread it around on the bottom of the pan.
Put a layer of eggplant (if using) in the pan; top with a layer of half the meat sauce, half the spinach, and half of the ricotta mix , sprinkle on half the mozzarella.
Repeat the layers.
Bake in preheated 400 degree oven until the lasagna is bubbling and the cheese is melted and lightly browned on top, about 30 minutes.