Preheat oven to 350˚F (180˚C).
In a medium bowl, add rhubarb, half of the sugar, half of the vanilla, orange zest, and juice, and toss to combine. Butter a baking dish and lay rhubarb in a single layer. Bake for around 10 minutes, or until just soft.
Remove from the oven, add raspberries, toss gently, and leave to cool.
Butter a 9-inch or 10×7-inch baking pan (gratin dish).
In a large bowl, whisk the eggs with the rest of the sugar and vanilla until foamy. Add the milk, heavy cream, and melted butter, whisking again to combine. Whisk in the flour and salt and mix until completely smooth. Combining the ingredients in a blender will make your batter even smoother.
Spread rhubarb mixture in prepared pan and pour over custard mixture.
Bake for 40-45 minutes, or until the edges and top of the clafoutis are golden and the middle is just set.
Remove from oven and cool for 5 to 10 minutes. Dust with powdered sugar and serve with vanilla bean ice cream.