Peel the apples and cut all the fruit into small pieces.Place in a saucepan.
Cut the ends of the rhubarb stalks and peel them to remove the strings.Cut them into pieces of about 1 centimeter and add them to the apples in the pan.Add 50 g of brown sugar and the lemon juice.
Cook over medium heat for 10 minutes, stirring constantly. Add the strawberries and cook for another 5 minutes, stirring.Pour into a baking dish.
Preheat your oven to 200°C/390°F.
Prepare the crumble by pouring in the flour, brown sugar and butter and sanding with your fingers until the crumble forms.Add the oatmeal and stir with a fork.
Pour the crumble batter over the fruit and bake for 30 to 40 minutes, until the crumble is golden brown.
Serve warm or cold, depending on your taste.
Notes
If you like sweet desserts, feel free to add more sugar with the fruit. I tend not to put a lot of sweetness in my desserts and I know that it can sometimes be unpleasant...