Preheat your oven to 425°F (220°C). Place a glass baking dish (about 9×13 inches) on the middle rack while the oven heats so the dish gets hot; this helps the radishes start to caramelize as soon as they hit the pan.
Prepare the radishes: Rinse the radishes under cool water to remove any grit. Trim off the root tips and leafy tops, then cut each radish in half from top to bottom. If you have any very large radishes, cut them into quarters so all pieces are roughly the same size for even roasting.
Season the radishes: In a large bowl, toss the halved radishes with the olive oil (or melted butter), kosher salt, and dried Italian herb blend until every piece is lightly coated and the herbs are distributed evenly.
Transfer to the hot dish: Carefully remove the hot glass baking dish from the oven, using oven mitts. Quickly spread the seasoned radishes in an even layer, cut sides down and skins facing up when possible. This encourages tender centers with crisp, wrinkled skins.
Roast and caramelize: Place the dish back in the oven and roast for 20 minutes. Stir gently, then spread the radishes out again, keeping most cut sides down. Continue roasting for another 15–20 minutes, or until the centers are very tender when pierced with a fork and the skins are golden brown and slightly wrinkled.
Finish and serve: Taste a radish and adjust the salt if needed. Serve hot, straight from the glass baking dish, making sure to scoop up any herby, savory oil pooled at the bottom and spoon it over the radishes for extra flavor.