Preheat the oven to 375 degrees F with the cast iron skillet inside.
Roughly chop the rhubarb into small pieces.
In a mixing bowl, combine the brown sugar, flour, oats, and cinnamon.
Using a cheese grater, grate 1/2 cup of butter into the bowl. Mix well until crumbly.
Once the oven and skillet are heated, carefully remove the skillet from the oven. Place 1/4 cup of butter into the skillet and allow to melt.
Add the rhubarb to the skillet and spread evenly along the bottom. Drizzle the honey all over the rhubarb pieces. No need to mix.
Sprinkle the topping evenly all over the rhubarb pieces. Place a couple of pads of butter on the top, if desired, for extra shine.
Bake for about 30 minutes, or until golden brown and bubbly.
Serve warm with a scoop of ice cream on top, if desired.