Wash the radishes and trim their ends.
In a medium saucepan, toast the cumin seeds on medium heat until fragrant, about 30 seconds. Add the radishes, broth, salt and pepper to the pan, and bring to a gentle simmer. Simmer on low heat with a closed lid until the radishes are tender, about 8 to 10 minutes. Remove the radishes from the pot and set aside the cooking liquid.
In a medium skillet, heat the olive oil over high heat. When hot, sauté the radishes until lightly browned on all sides, about 2 minutes per side.
Put the saucepan with the cooking liquid back on the stove and cook on high heat until the liquid reduces by half, about 3 minutes. Add the radishes back in, if desired.
Whisk in the butter with a wire whisk, then add the lemon zest. Serve the radishes warm, drizzled with the reduced butter sauce.