1. Preheat oven to 350 degrees Fahrenheit.
2. In a small bowl, combine the melted butter and brown sugar. Stir to combine. 
3. Add the dry ingredients and mix until well combined. 
4. Press half of the crumble mixture into the base of your oven safe skillet or 9x9 glass baking dish.
5. For frozen fruit only: slightly thaw your rhubarb and drain excess liquid from it. Completely thaw peaches and place them on a paper towel to absorb excess juices. Then toss together in a bowl with an additional 1/2 cup of flour.
6. Combine granulated sugar and starch and place in a small pot. Pour boiling water over the sugar mixture and stir until thick, cooking over heat if needed. Do not over thicken. Remove from heat and add vanilla.
7. Place fruit mixture over the crumble base and pour the sugar syrup evenly over the fruit.
8. Top with remaining crumble mixture and bake at 350 degrees Fahrenheit for 30-40 minutes or until fruit is bubbling and the top is golden brown. 
9. Allow the crisp to rest for 15-20 minutes before serving. Top with ice cream if desired.

*All nutritional information is an estimation only. Exact nutritional value depends on exact amounts and ingredients used.