Put all ingredients in a bowl. Cut the butter with a knife and crumble the flour and sugar with the butter until a sandy structure. Stop mixing when the dough tends to stick together.
Sprinkle the dough over the rhubarb and strawberries and bake in about 40 minutes. Check from time to time the colour. It should be brown, not black.
Serve with creme fraiche and a few fresh strawberries, shortly held in a sugary liquid. Preferably with a flavour of lavender.