Halve the radishes or quarter any that are larger than a half dollar.
Cut the onions or shallots into wedges the size of the radish pieces.
Pile onto a rimmed cookie sheet or cast-iron skillet.
Drizzle the oil over the vegetables, sprinkle the salt and black pepper over the top and toss to coat, spreading the radishes and onions out into a single layer.
Roast for 10 minutes, lower the heat to 350°F, stir the vegetables, and roast for 20 to 30 more minutes, stirring every 10 minutes, or until the vegetables are deeply caramelized on the outside and tender on the inside.