I cut it into cubes about 1" after it has been peeled, and put it in a bowl in the microwave with a little seasoning. Cover and cook for about 5-8 mins, depending on quantity. Test to see if it is soft, and cook a bit more if necessary. Put into a sieve to drain out all the water, and back into the bowl. Mash well, and drain again if necessary. Test seasonings and serve as is.
NB There is a lot of liquid in marrow so you definitely do not want to add any more.
This may be a bit late..but have only just logged in...Try it peeled, cut into cubes, placed in pan, tiny bit of water to stop it burning, heat gently with salt, pepper, and a tablespoon of Marmite. ...strange as it may seem, it adds a certain something!!!! Stir round well & simmer gently till its done. Being a veggie, will go down well. Enjoy!!
I've grown several large marrows this year, must be the weather for them. They have been too big for the two of us so we've resorted to that old favourite stuffed marrow using home made stuffing of various ingredients including ginger and peppers. Having cooked the marrow, sliced down the middle and then cut into portions, we eat half and freeze the other half, a ready meal, just heat it up. Personally I like to sprinkle with soy sauce.