Prep the fruit. Put the rhubarb and apples in a large bowl. Toss with the sugar, flour, and cinnamon (and zest, if using) until evenly coated. Tip into a 30 x 20cm (12 x 8") baking dish.

Make the crumble base. In a large bowl, combine the topping flour, sugar, brown sugar, and salt. Add the cold, diced butter and rub it in with your fingertips, lifting and rubbing the mixture between your fingers, until it looks like coarse breadcrumbs with some pea-sized lumps.

Add oats and almonds. Stir in the oats, and work with your hands, squeezing bigger lumps in your whole hand and then breaking it up. You want varied crumble lump sizes, around pea-size. Not dusty, sandy texture. Then mix in half the flaked almonds (60g), keeping the rest for the top.

Assemble and bake. Spread the topping evenly over the fruit, then scatter the remaining almonds on top. Bake at 190°C / 375°F (170°C fan) for 45-50 minutes, until the topping is golden and the fruit is bubbling at the edges.

Rest before serving. Let the crumble sit for at least 15 minutes before serving - it will thicken slightly as it cools. Serve with ice cream, custard, or cream.