Peel the sweet potato and slice into even size discs.
Boil for 20 minutes or so until fork tender
Meanwhile, preheat the oven to gas 4 / 180C / 350F and lightly grease a casserole dish
Put the butter and flour in a bowl and rub it together with your fingers until it resembles breadcrumbs.
Mix the sugar in and the pecans if using and it's ready.
Set aside
Drain the potatoes and add in the sunflower spread, sugar, vanilla and pumpkin pie spice.
Mash together well to combine
Stir in the marshmallows.
*most store bought brands are free-from gluten, dairy and eggs but not vegan as they use either pork or beef gelatine but vegan mallows are widely available from health food shops and online
Spoon the mixture into the casserole dish, smoothing the top down
Sprinkle the crumble topping over evenly then bake for 35 - 45 minutes until golden on top