Preheat the oven to 190C/375F.
Chop the rhubarb sticks and slice the strawberries. In an oven proof dish, add the rhubarb, strawberries, chia seeds and lemon juice. Mix well and spread all over the dish.
Chop the walnuts. In a small bowl, combine the butter, stevia, cinnamon, almond flour, oat fiber and walnuts. Mix until it resembles a crumble.
Place the crumble over the fruits and put in the oven for 30 minutes.
Take out and eat hot or cold!