Add rhubarb to a large cast iron skillet or saucepan, bring to medium heat, and soften the rhubarb, stirring often, so as not to burn the pan (about 10 minutes). When the rhubarb has softened, add raspberries and honey. Bring compote to a low simmer and add vanilla, salt, lemon juice, and arrowroot powder. Let simmer until thickened and the juices have reduced by almost half (about 15 minutes).

In a mixing bowl, combine oats, almond flour, walnuts, coconut sugar, and salt. Pour melted coconut oil over the mixture. Mix well to combine.

Pour the compote into a baking dish (if using a cast iron skillet, skip this step). Sprinkle the crumble mixture evenly over the top of the compote.

Preheat the oven to 350°F. Bake for 30-45 minutes or until golden and bubbling.