Seared Alaskan halibut with mashed potatoes & herbed brown butter-parmesan sauce

• Scrub the potatoes. Cut the potatoes into 1-inch pieces.
• Set aside half the garlic-Parmesan butter for the pan sauce.

In a medium [large] sauce pot, add the potatoes and enough lightly salted water to cover by 2 inches. Bring to a boil, reduce to a simmer, and cook until tender, 10 to 12 minutes. Drain the potatoes, reserving 1 cup [2 cups] of the cooking water in a medium heatproof bowl.

Return the potatoes to the pot along with ⅓ cup [⅔ cup] reserved cooking water and half the garlic-Parmesan butter. Using the back of a spoon or a masher, coarsely mash the potatoes, adding more cooking water as needed to reach the desired consistency (set aside the remaining cooking water for the pan sauce). Season to taste with salt and pepper. Cover and keep warm.

While the potatoes are cooking, prepare the remaining ingredients and cook the halibut.

• Finely chop, press, or grate the garlic.
• Juice half the lemon; cut the remaining half into wedges for garnish. [Juice 1 lemon; cut 1 lemon into wedges.]
• Strip the sage leaves from the stems; coarsely chop the leaves.
• Rinse the capers.
• Pat the halibut dry with a paper towel; season lightly with salt and pepper.

In a large frying pan, warm 1 to 2 tablespoons oil over medium-high heat until hot but not smoking. Add the halibut and cook until lightly browned on the bottom side, 4 to 5 minutes. Turn and cook until the flesh is opaque and flaky, 2 to 4 minutes. Transfer to a plate.

While the halibut is cooking, wilt the spinach and make the sauce.

In another large frying pan, warm 1 to 2 teaspoons oil over medium-high heat until hot but not smoking. Stir in the garlic and spinach (the spinach in batches if needed), season with salt and pepper, and cook until the garlic is fragrant and the spinach is just wilted, 1 to 2 minutes. Transfer to a plate. 

In the same pan over medium heat, melt the remaining garlic-Parmesan butter until fragrant and starting to turn golden brown, 2 to 3 minutes. Remove from the heat and stir in the sage and pine nuts, scraping up any browned bits from the bottom of the pan, until the sage and nuts are fragrant, about 1 minute. Stir in the lemon juice, capers, and 1 to 2 tablespoons of the reserved cooking water if desired for a thinner consistency. Season to taste with salt and pepper.

Transfer the halibut, spinach, and mashed potatoes to individual plates. Drizzle with the pan sauce and serve the lemon wedges on the side.