Preheat the oven to 500°. Trim the radishes and wash the greens; pat dry.
In a large ovenproof skillet, heat the oil until shimmering. Add the radishes, season with salt and pepper and cook over high heat, stirring occasionally, until lightly browned in spots, about 2 minutes. Transfer the skillet to the oven and roast the radishes for 15 minutes, until crisp-tender, stirring occasionally.
Return the skillet to the burner and stir in the butter to coat the radishes. Add the radish greens and spinach leaves and cook over moderate heat until they are wilted, about 2 minutes. Add the lemon juice and season with salt. Serve the radishes right away.