Ingredients

• 1/2 medium onion, unpeeled, halved
• 1/2 head garlic (halved crosswise)
• 1 lemongrass stalk, trimmed, tough outer layers removed, finely chopped
• 4 sprigs thyme
• 1 tablespoon black peppercorns
• 4 cups low-fat milk
• 1 1/4 pounds Yukon Gold potatoes (about 4 large), scrubbed
• Flaky grey sea salt or kosher salt
• 1/4 cup plus 3 tablespoons olive oil
• 1 1/2 pounds skinless cod fillet, cut into 2" pieces
• 1/2 preserved lemon, finely chopped
• 1/2 cup finely chopped chives (from about 1 large bunch)
• 2 tablespoons fresh lemon juice
• 1 tablespoon hot smoked Spanish paprika

Instructions

1. Bring onion, garlic, lemongrass, thyme, peppercorns, and milk to a boil in a large saucepan. Reduce heat to low and simmer 20 minutes. Strain infused milk into a medium saucepan. Discard solids.
2. Meanwhile, cook potatoes in a large pot of boiling salted water until tender, 25–30 minutes. Drain, then peel potatoes and coarsely mash in a large bowl with 1/4 cup oil; season with salt.
3. Bring infused milk just to a boil. Add cod, reduce heat, and simmer until flesh begins to flake and is cooked through, about 5 minutes. Remove cod with a slotted spoon and transfer to a plate.
4. Combine preserved lemon, chives, lemon juice, paprika, and remaining 3 tablespoons oil in a small bowl; season relish with salt.
5. Mix 1/4 cup infused milk into potatoes. Place cod over potatoes; spoon relish over.
6. DO AHEAD: Relish can be made 2 days ahead. Cover and chill.