Preheat the oven to 180°C (350°F).
In a 20 cm square pan, layer the rhubarb, strawberries and apple slices.
Sprinkle 125 ml of sugar over the fruits.
Drizzle with lemon juice.
Sift the flour, baking powder and salt.
Add the flour. Set aside.
With the mixer, combine butter, sugar, eggs and vanilla until the mixture is very smooth.
Gradually add the dry ingredients, alternating with the milk.
Spread the dough over the fruits mixture.
Cook in the oven at 180°C (350°F) for about 30 minutes.
Serve warm or cold with whipped cream.
Yield: 6 to 8 servings.