Step 1

Preheat oven to 325 degrees F.

Step 2

Heat a large cast-iron skillet over high heat. Add onions and cook, pressing down occasionally, until completely charred on one side, 10 to 15 minutes. Turn and cook, pressing down occasionally, until charred on the second side, 8 to 10 minutes more. Transfer to a large bowl and cover tightly. Let steam for 15 minutes.

Step 3

Meanwhile, mix 2 tablespoons oil, lemon zest, pepper and 1/2 teaspoon salt in a small bowl. Add 1 tablespoon oil to a medium ovenproof nonstick or cast-iron skillet and add salmon, skin-side down. Spread the lemon zest mixture on the salmon. Bake until just cooked through, 14 to 16 minutes.

Step 4

Bring a small saucepan of water to a boil. Add radish wedges and cook until just tender, 5 to 6 minutes. Drain and pat dry. Dry the pan and add the remaining 1 tablespoon oil and Old Bay. Heat over medium heat until sizzling, about 1 minute. Remove from heat and stir in the radish wedges. Cover and keep warm.

Step 5

Combine the onions, the remaining 1/2 teaspoon salt and sugar in a food processor; puree until smooth.

Step 6

Serve the salmon with the onion puree and radishes. Garnish with sliced radishes, cilantro and flaky salt, if desired.