You can roast them whole wrapped in silver foil. Pierce the skin in several places before you wrap it
If you are frying/grilling/baking them sliced the flesh can be a little bitter, sprinkling liberally with salt and leaving for 20 minutes (known as "degorging") will remove much of the bitterness and draws out some moisture - you may find that helps with the toughness too.
Bake/Roast Whole = 35-40 minutes - pierce the skin in several places
Slices/chunks 20-30 minutes - brush with oil and season
Boil Chunks = 20-30 minutes
Fry/Sauté Slices/chunks 8 - 12 minutes Use very hot fat/oil to seal quickly
Microwave Slices/chunks = 4 - 6 minutes
Steam Slices/chunks 20-30 minutes
Stuff Halves - +60 minutes in total (see individual recipes)
Moussaka Slices as a topping for the mince mixture and covered with HEA cheese (see recipes)
Hope this helps.