Preheat oven to 400F. Move the rack to the middle position.
Cut the radishes into equal-sized pieces. If they're particularly large radishes, cut them into quarters. Otherwise, cut them into halves.
Add the radishes to a baking dish and toss them with the olive oil and salt & pepper. Roast them face down if you want them to brown nicely.
If the radishes are small, roast them for 15 minutes and check to see if they're done to your liking. If they're larger, it may take up to 35-40 minutes to get them tender. Mine took about 30 minutes to get tender-crisp.
Toss with freshly chopped parsley if desired.
Notes
Calorie information is provided as a courtesy only and should be construed as an estimate rather than a guarantee.