Cut and peel yam, cut into 1-inch cubes and rinse in a bowl of water.
Grind together tomatoes, tatashe, shombo, scotch bonnet and 1/2 of the onions.
Slice the other half of the onions and set aside.
Place yam in a pot, add 2 cups of water and stock (if using), place over medium-high heat.
Add ground tomato mix, crayfish, seasoning cubes and a little salt.
Add palm oil and sliced onions.
Cover and cook till the yam is very soft and the liquid very much reduced.
Use a potato masher or wooden spoon and mash the pieces of yam, leaving some whole.
Reduce the heat and cook for about 5 minutes, stirring from time to time.