Wash the rhubarb thoroughly, chop it into small pieces, peel and dice the pears, and combine them in a large mixing bowl with sugar, lemon juice, vanilla extract, and ground cinnamon.

In another bowl, mix together the gluten-free rolled oats, almond flour, remaining sugar, melted butter, and salt until crumbly.

Pour the fruit mixture into a greased 9x13 inch baking dish and spread it evenly, then sprinkle the crumble topping over the fruit layer.

Preheat the oven to 350°F, bake for 35-40 minutes until golden brown, and let cool for 10 minutes before serving.