Lightly butter an 8 or 9-inch square baking pan. Preheat the oven to 375℉.
Make the topping: In a medium bowl, mix together the 1 ½ cups old-fashioned rolled oats, ½ cup oat flour, ½ cup sugar, ½ teaspoon fine sea salt, and ½ teaspoon ground cinnamon. Drizzle in the 8 tablespoons unsalted butter and stir until it is incorporated.
Make the filling: In a large bowl, whisk ½ cup honey and 2 tablespoons arrowroot, then add the 3 cups chopped rhubarb3 cups chopped strawberries, and 1 tablespoon lemon juice. Stir and toss gently to coat, then spread the filling out evenly in the baking dish.
Place in the oven and bake until the topping is golden and firm and the the fruit juices are bubbling around the edges, about 45-50 minutes. Remove to a wire rack and let cool for at least 15 minutes before serving. Crisp may be served warm or at room temperature.
Notes
Cut the rhubarb evenly- Uniformly cut pieces of rhubarb help it cook at about the same rate as the strawberries, making for an evenly cooked and tender filling.
Taste the strawberries - If your strawberries are extra sweet, you can reduce the honey slightly; if they’re more tart, feel free to add a bit more. Or, sweeten to your taste with more or less honey. Use my measurement as your jumping-off point.
Let it cool- Rest the crisp after baking so that the filling thickens up a bit. This wil ensure a thick and jammy filling that's not goopy.
Oats and oat flour substitute - Substitute almond flour and sliced almonds for the oat flour and oats, or use 1 ½ cups of your favorite 1:1 gluten-free flour blend with sliced almonds.
Make it dairy-free - Substitute your favorite non-dairy butter for the butter or melted coconut oil for the butter.