Heat up butter in a cast iron skillet on medium heat until its completely melted.
Add in radishes and red onion and stir over medium heat for 10-15 minutes to caramelize.
Stir in the minced garlic and cook for 2-3 minutes until fragrant.
Toss in the radish leaves and kale. The skillet might be overflowing at this point, but that's okay, the leaves will wilt and it will shrink down to a manageable size. Cook for another 2 minutes until the leaves are wilted.
Remove and serve! Or if it's just you feel free to crush directly from the skillet.