Start by cutting rhubarb stems into 1 to 2 cm sections. If rhubarb is a bit firm, peel of the outer thin skin very thin. In a bowl, put rhubarb and sugar, combine and leave to macerate for at least an hour.
Prepare the clafoutis batter
Preheat your oven to 180°C/350°F.
Melt butter un a double boiler or in the microwave.
In a bowl, beat the eggs, and add the sugar, continue beating.
Then add flour and a pinch of salt, combine.
Finally add milk and the melted butter. Mix until you get a homogeneous texture.
Drain rhubarb. Rinse red fruits and remove stems (cut strawberies in half if they are big, or leave the raspberries whole).
Bake
Grease the baking dish with butter.
Pour fruits in the dish and cover with clafoutis batter.
Bake in the oven at 180°C/350°F for 30 to 40 minutes, depending on the thickness.