Cut the large radishes into quarters, the medium ones in half, and keep the small ones whole. Slice the onion in half, then each half into quarters, and each quarter in half.
Spread the vegetables on a large baking sheet. Drizzle the olive oil over the top. Sprinkle the herbs and salt evenly over the top. Toss everything together until well coated in the oil.
Roast at 425°F for 15 minutes, or until the radishes are fork-tender.