Wash the radishes and trim off the greens, saving a few leaves for a garnish.
Cut the radishes lengthwise. Toss them in a bowl with the olive oil, finely grated garlic clove, kosher salt and pepper.
Line a baking sheet with parchment paper. Place the radishes on top cut-side down and roast for 16 to 20 minutes until the largest radish is tender when pierced with a fork at the thickest point (exact time depends on the size of the radishes).
Very finely chop about 1 radish leaf into a garnish, and sprinkle over the top. Serve warm.