¼ medium sweet onion (preferably Vidalia)
1 tablespoon plus 2 teaspoons red wine vinegar
½ teaspoon finely chopped fresh thyme leaves
¼ teaspoon kosher salt or coarse smoked salt
1/8 teaspoon freshly ground black pepper
1/3 cup extra-virgin olive oil
18 small radishes, divided
16 marble potatoes, halved (or 8 new potatoes, quartered)
2 tablespoons canola oil
½ teaspoon kosher salt
¼ teaspoon freshly ground black pepper
2 tablespoons trout or whitefish roe (optional)
2 sticks (16 tablespoons) unsalted butter
½ pound radishes (preferably large radishes), peeled and finely chopped
2 tablespoons whole milk
½ teaspoon kosher salt
¼ teaspoon freshly-ground black pepper
1. Make the vinaigrette: Adjust the oven rack to the upper-middle position and preheat the broiler. Place the onion quarter on a square of aluminum foil and set on a baking sheet. Broil until the onion begins to char, about 15 minutes (check the onion often, as broiler intensities vary). Once the onion is lightly charred, remove the baking sheet from the oven and wrap the onion in the aluminum foil. Reduce the oven temperature to 400° and adjust the oven rack to the middle position.
2. Make the salad: Thinly slice 2 radishes into rounds and place them in a bowl of ice water. Halve the remaining radishes and place them in a large bowl. Add the potatoes, canola oil, salt and pepper. Transfer the mixture to a rimmed baking sheet and add the foil-wrapped onion. Roast until the radishes and potatoes are golden and tender, about 25 minutes. Remove from the oven.
3. While the radishes and potatoes roast, make the radish purée: In a medium saucepan set over medium-low heat, melt the butter. Add the radishes and enough water to just cover them. Once the liquid comes to a simmer, reduce the heat to low and gently simmer until a fork easily slips into a radish, about 30 minutes. Strain the mixture into a fine-mesh sieve set over a medium bowl. Place the cooked radishes in a blender and add the milk, salt and pepper. Purée until smooth, then transfer the purée to a medium bowl and cover with plastic wrap to keep warm. Rinse out the blender before finishing the vinaigrette.
4. Finish the vinaigrette: Unwrap the onion and add it to a blender along with the red wine vinegar, thyme, salt and pepper. Blend on low speed while slowly adding the olive oil through the opening in the lid, increasing the speed as the vinaigrette becomes thin and fluid. Blend until the mixture is completely smooth and emulsified.
5. Remove the sliced radishes from the ice water and place on a kitchen towel to dry. Divide the warm radish purée among 4 plates. Top with the roasted radishes and potatoes and drizzle with the vinaigrette. Top with the radish slices and a dollop of trout roe, if using. Serve immediately.