Butter a 9x9 in. square pan and preheat oven at 375°F (200°C)
Toss the rhubarb with sugar, lemon juice, cornstarch, and any spices in a large pot and cook on low heat for 10 minutes.
Pour this mixture in the square pan.
Add raspberries with their juice and mix delicately.
In a bowl, mix brown sugar and sift flower over it.
With 2 knives cut in the butter and quickly work into a grainy texture with your hands.
Sprinkle over the fruit and cook approximately 30 minutes until the fruit juices are bubbling around the edges of the pan and the topping is firm to the touch.
Let the crumble cool for at least 15 minutes before serving. If transporting to a picnic or party, let the crumble cool completely to give the fruit filling time to set.