Peel, core, and slice the apples into 1/2-inch wedges, then trim the rhubarb and cut into 1-inch pieces. Set both aside in separate bowls. This prep work takes just a few minutes and ensures everything is ready when you need it, which is especially important since the fruit cooks quickly once the heat is applied.

In a bowl, combine the cold cubed margarine with the gluten-free flour using your fingertips or a pastry cutter, working quickly to keep everything cold—this creates those desirable crumbly pockets. I find that cold margarine is absolutely key here; if it gets warm, you'll end up with a more cake-like texture instead of the satisfying crumble you want. Stir in the dark brown sugar, cinnamon, oats, and salt until just combined. Set the mixture aside at room temperature.

Melt 1 oz margarine in a large pot over medium heat and add 2 oz brown sugar, stirring for about 2 minutes until the mixture becomes caramel-like and fragrant. Add the sliced apples along with 1/2 teaspoon cinnamon and 1/4 teaspoon ground ginger, cooking for 2-3 minutes until the apples begin to soften slightly. Then add the rhubarb and cook for another 2-3 minutes, stirring gently to combine. The fruit should release some juice but still hold its shape—you're looking for a balance between tender and structured.

Preheat your oven to 180°C (350°F). Transfer the warm fruit mixture into a 9x13 inch baking dish or similar-sized dish, spreading it evenly. Distribute the crumble topping from Step 2 evenly over the fruit, pressing it down very gently—don't compact it too much or you'll lose that desirable texture. Bake for 25-30 minutes until the crumble topping is golden brown and the fruit is bubbling slightly at the edges.

Let the crumble cool for 5-10 minutes straight from the oven—this allows the fruit to set slightly while the topping stays crispy. Serve warm with custard or ice cream for a classic, comforting finish.