Set the oven at 375°F. For the fruit filling, put rhubarb and strawberries in an 8 by 10-inch ceramic dish.
Melt butter in a sauce pot on medium heat. Make a roux by adding oat flour and cook while whisking for 1 minute.
Add brown sugar and water to the pot and bring to a boil while whisking. Once boiled, carefully pour over the strawberries and rhubarb. Mix well.
For the crumble, melt the butter. Use a wooden spoon to mix it with all other crumble ingredients in a bowl. Evenly sprinkle crumble over the fruit mix.
Put the ceramic dish on a baking sheet to catch any drips and bake for 25 to 30 minutes. Let cool slightly before serving.
