Pre-heat the oven to 350º F, and grease a 9x13 baking dish with cooking spray.
In a large mixing bowl, stir together the rhubarb and strawberries.
Add the sugar, flour, lemon juice, and vanilla extract, stirring to combine.
Pour the mixture into a greased baking dish.
In a medium mixing bowl, whisk together the brown sugar, flour, oats, cinnamon, nutmeg, and nuts (if using).
Stir the melted butter into the flour mixture until everything is combined and crumbly.
Crumble the mixture on top of the fruit.
Bake at 350° F for 35-40 minutes, or 'til the crisp is golden brown and the fruit is bubbling.
Cool for 10-15 minutes before serving. Serve it with whipped cream or vanilla ice cream, and/or enjoy with a piping hot cup of tea or coffee.