Preheat oven to 375°F. Grease a 6 or 8 cup casserole dish and set aside.
Carefully wash raspberries and put on a paper towel or in a colander to drain. Peel rhubarb, similar to celery, by taking a knife and pulling strings if desired. Slice into ⅛ inch slices (no need to be exact).
Place rhubarb into baking dish and top with raspberries. Sprinkle with sugar and flour, tossing to coat. Set aside.
To make topping, using a pastry blender, mix together the butter, oats, flour cinnamon, and sugar. Once fully mixed, use a spoon to stir in coconut or nuts if using. Spoon topping on top of fruit.
Place baking dish on a baking tray (line with parchment in case of any spills).
Bake 35 minutes, until bubbling on the edges. Check if rhubarb is done by using a fork or knife in the center. If it starts to brown but the fruit is not ready, cover loosely with foil or parchment and bake for 5 more minutes. If still not ready, check after 3 minutes until it is cooked to your liking.
Let cool for 5 minutes before serving to thicken the fruit mixture.
Notes
You may use 5 cups of any fruit mixture. If not using rhubarb, cut sugar to 2-3 Tablespoons and flour to 1 Tablespoon. If using frozen fruit, defrost and do not drain fruit.Smart tip: Double the topping and put half in a Ziploc bag in your fridge (or freezer). It will keep for a couple of weeks. Simply add to your next fruit mixture and bake as directed!