Wash the rhubarb sticks, cut off the ends and remove a few fibres (you can also leave them). Cut them into 2 to 3 cm pieces.
Cook them for 5 minutes in a saucepan over low heat with 3 tablespoons of brown sugar and about 50 mL of water. Set aside.
Wash the strawberries, remove the stalks, and cut them in 2 or 4 according to their size.
Wash and dice the apples.
Place the strawberries, apples and rhubarb in a rectangular glass dish (I use a dish about 20 cm long). Sprinkle with vanilla powder.
Then prepare the crumble: mix the flour and sugar in a bowl.
Then add the chilled butter cut into cubes (take the butter out of the fridge at least 1 hour beforehand). Mix with your fingertips until you obtain a fairly homogeneous "sandy" texture.
Spread the crumble over the fruits, and put in the oven for 30 to 35 minutes at 180°C.
Let it cool before eating. You can eat your crumble warm or cold, and serve it with a scoop of ice cream or whipped cream :)