Preheat the oven to 375°F. Toss the rhubarb and strawberries together in a large bowl with the sugar. In a small cup, mix together the cornstarch and orange juice. Set these both aside while you make the topping.
In a medium bowl, mix the oats, gluten-free flour, ground almonds, flaxseed and salt. Add the melted coconut oil, syrup and vanilla and stir till the dry ingredients are moist and crumbly. Mixture should hold together when pinched between two fingers.
Drain any excess liquid from the fruit, then pour in the orange juice mixture and stir. Empty into an 8-inch square baking dish. Sprinkle the topping over the fruit and place the dish on a baking tray lined with parchment paper. Bake for about 25 to 30 minutes, until the fruit is bubbling around the edges and the topping is golden brown. Best served warm.