Preheat oven to 375°F. Grease a 6 or 8 cup baking dish & set aside.
Peel rhubarb, similar to celery, by taking a knife and pulling strings off (if desired). Slice into ⅛ inch slices (no need to be exact).
Place rhubarb into baking dish. Sprinkle with sugar & flour, tossing to coat. Set aside.
To make topping: using a pastry blender, mix together the butter, oats, flour, cinnamon, & 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 cookie sheet or tray (line with parchment in case of any spills).
Bake 30-35 minutes, until bubbly on the edges. Check if rhubarb is done by using a fork or knife in the center. If topping 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.