Preheat the oven to 375°F.
In a medium pot with a steamer basket, add 1 inch of water and the potatoes. Bring to a simmer, cover, and cook for 8 to 10 minutes, or until fork-tender. Drain the potatoes and let cool slightly.
Meanwhile, cook the bacon in a large skillet over medium heat until crisp. Drain on paper towels and roughly chop when cool.
In a large mixing bowl, combine the onion, parsley, scallions, chives, sherry vinegar, mayo, salt and pepper. Fold in the potatoes and bacon and adjust seasoning.
On a baking sheet, toss the asparagus with the oil, salt and pepper. Roast for 10-15 minutes, until crisp-tender. Remove from the oven and toss with the lemon juice.
Drizzle the fish with olive oil and sprinkle liberally with salt and pepper, including the cavity.
Heat a grill, large ovenproof or other skillet over high heat. Add the branzini and cook for 3 minutes per side, until the skin is crisp. Transfer to the oven (using a baking sheet if you don't have an ovenproof skillet) and roast for 5-8 minutes, until cooked through.
Divide the potato salad and asparagus between plates. Top with branzini.