Wash, halve, and pit the plums. Cut them into wedges and place them in a pie dish or an ovenproof dish. Mix raw sugar with cinnamon and ginger and sprinkle it over the fruit in the dish. Toss the plums so that the spiced sugar coats them evenly.  

Crumble:

Mix oats, wheat flour, honey, and syrup in a bowl.  Add the butter and knead it into the mixture with your hands until it becomes a crumbly dough. Distribute the crumble over the plums and bake the cake for 20-30 minutes at 180 degrees Celsius (356 degrees Fahrenheit) until it is crisp and golden.   Serve the cake with crème fraiche stirred with vanilla sugar or enjoy it with vanilla ice cream.