Start by making the plum crumble topping - preheat the oven to 200°C / 400°F / Gas Mark 6. Place the halved plums skin-side-down in a greased oven-proof dish, (try to tuck them as closely together as possible, so use a fairly small dish). Scatter them with 1 tbsp sugar, then the crumble mix and oats. Bake for 30 minutes or until the crumble is golden and crispy.
Meanwhile, make the waffles - combine in the flour, baking powder, sugar and salt in a bowl. Next, in a mixing jug combine the milk, vanilla extract, banana and melted margarine. Make a well in the centre of the dry ingredients, and pour in the wet ingredients. Mix with a for for a minute or too until smooth.
Heat up the waffle maker. Pour in one-third of the batter, close the lid and cook until ready. Repeat for the final two waffles. Slice them into quarters, plate up two quarters per serving, and top each with half a crumble-topped plum. If you want to make the waffles ahead, just cook a little less (so they are a little paler than usual), then pop the quarters in the toaster for a minute before serving. This would be lovely with a scoop of vegan ice cream or drizzle of soya custard!