Trim radish tops and root tips; halve larger radishes lengthwise so pieces are roughly even (about 20 medium radishes total). Pat dry with paper towel.

Heat a large heavy skillet (cast-iron preferred) over medium-high heat until hot, about 2 minutes.

Add 1 tablespoon olive oil and 2 tablespoons butter to the skillet. Allow butter to foam and just begin to brown — this helps develop nutty flavor.

Place radish halves cut-side down in a single layer. Cook undisturbed for 4–6 minutes until the cut faces are deeply golden brown and caramelized; reduce heat to medium if butter is browning too quickly.

Sprinkle 1 teaspoon granulated sugar over the radishes during the last minute of searing to encourage additional caramelization. Continue to cook 1 minute.

Flip radishes with tongs and cook 1–2 minutes on the rounded side until just tender when pierced with a paring knife (they should retain slight bite).

Season with 1 teaspoon kosher salt and 1 teaspoon freshly ground black pepper while still in the pan. Sprinkle chopped parsley and 1 tablespoon lemon juice over the radishes; toss gently to coat. Transfer radishes to a warm plate and set aside.

Caramelized Tonnato–Horseradish Sauce

While radishes cook, make the tonnato-horseradish sauce. In a food processor or small blender combine 3 tablespoons mayonnaise, 60 g drained tuna, 2 chopped anchovy fillets, 1 teaspoon rinsed capers, 1/2 teaspoon lemon zest and 1 teaspoon lemon juice.

Pulse to a coarse paste (do not over-blend — you want some texture). Scrape down the sides.

Add 1 teaspoon prepared horseradish, 1 teaspoon lightly toasted breadcrumbs, 1/2 teaspoon brown sugar and 1 teaspoon olive oil. Pulse briefly to combine. If the sauce is too thick, add up to 1 teaspoon water and pulse again to reach a spreadable, slightly loose but creamy consistency.

Taste and adjust seasoning: add a touch more horseradish for heat, a pinch more sugar if too tangy, or another small squeeze of lemon if it needs brightness.

Transfer sauce to a small bowl. For a lightly caramelized flavor note, you can warm a small skillet over medium-low, add the 1/2 teaspoon olive oil and 1/2 teaspoon brown sugar, swirl until sugar begins to melt and smell caramel-like (30–45 seconds), then whisk this into the sauce. This step is optional but enhances the 'caramelized tonnato' idea.

Plating & Finish

Spread 1–1.5 tablespoons of the tonnato-horseradish sauce in the center of each serving plate (or as a smear across a communal platter). Arrange caramelized radishes on top of the sauce, cut sides up for visual contrast.

Drizzle 1/2 teaspoon cold-pressed olive oil over the radishes, sprinkle microgreens or additional chopped parsley, and finish with a light pinch of flaky sea salt.

Serve immediately while radishes are warm and the sauce is cool for the best contrast.