Preheat your oven to 375°F (190°C). Lightly grease an 8x8 inch baking dish.

In a large bowl, combine the chopped rhubarb, 0.5 cup erythritol, lemon juice, and vanilla extract. If using xanthan gum, sprinkle it over the filling and toss to combine.

Pour the rhubarb mixture into the prepared baking dish and spread evenly.

In a separate medium bowl, whisk together the almond flour, chopped pecans, 0.33 cup erythritol, cinnamon, and salt for the crisp topping.

Pour the melted butter over the dry topping ingredients and stir with a fork until the mixture is crumbly and well combined.

Sprinkle the crisp topping evenly over the rhubarb filling in the baking dish.

Bake for 30-40 minutes, or until the topping is golden brown and the rhubarb is tender and bubbly. If the topping starts to brown too quickly, you can loosely tent it with foil.

Let the crisp cool for at least 10-15 minutes before serving. Serve warm, perhaps with a dollop of keto whipped cream or a scoop of sugar-free ice cream.