Slice the eggplant vertically into 0.5-0.8mm slices. Arrange in a single layer, sprinkle evenly with a teaspoon of salt, then place the remaining slices on top and season with salt as well. The salt tenderises the flesh and makes the eggplant absorb less oil when cooking. It also reduces some of the bitterness that may be present in the eggplant (although most eggplants these days are not very bitter). 

Set aside for 10-15 minutes while you prepare the sauce.

Prepare the tomato sauce ingredients. Finely dice the onion, and garlic, drain sun-dried tomatoes and chop if needed. Grate the cheeses. Mozzarella can be replaced with other well-melting cheese.

Heat 1-2 tablespoons of olive oil in a deep frying pan or a pot and add the onion. Saute for 3-4 minutes until softened, then add the garlic, sun-dried tomatoes, chili flakes and 1 teaspoon of salt. Stir through for a minute and add the canned chopped tomatoes (2 cans). 

Stir and cook for 15-20 minute over medium heat. Keep it uncovered and stir frequently as we want to evaporate the watery tomato liquid and thicken the sauce. You should be able to draw a path along the base of the pot that remains visible for a few seconds. This process will also intensify the flavours.  

Once done, use an immersion blender and partially puree the sauce. I do this direclty in the pan. If you don't have a blende, it's okay to leave it chunky. 

This can be done in parallel to cooking the sauce. I use two frying pans to make the process a bit quicker. Heat the pans over medium-high heat and once really hot, add 1.5 tablespoons of olive oil to each. If your pan is very large, you might need 2 tablespoons.

Use some paper towels or a kitchen towel to gently pat-dry the top layer of the eggplants. This will absorb the water drops and some of the salt. Place a few slices of eggplant in each pan in one layer. Cook for 2-3 minutes on the first side, until golden brown and a bit shorter on the opposite side. Don't worry if the pan gets a little dry. Only add extra oil if it feels like it might be burning. If more oil is needed, drizzle just a little bit around the sides or in between the eggplants and swirl around. 

Remove cooked eggplant to a bowl or another pan and set aside. Continue until all eggplant slices are done. 

In the meantime, preheat the oven to 200 C / 400 F.

Time to assemble it all. Smear a quarter of the sauce on the base of a sheet pan or baking dish. Top with a layer of eggplant, I use about half of it, and some slices may overlap. 

Spread the eggplant with half of the remaining sauce. Layer the remaining slices on top and finish with the sauce. 

Finish with grated Mozzarella and Parmesan cheese on top. I ended up using 1.5 cups of Mozzarella thinking that it will need it but once it was baked all the cheese was melted, I think 1 cup of each is plenty unless you like it very cheesy. 

Pop in the oven, middle shelf, for 15-20 minutes until the cheese is completely melted and golden brown all over. 

Remove from the oven and let it cool for a few minutes before slicing and serving. It's even better after it sits for half an hour or so. It can absolutely be served barely warm or cold.