How to Make It
Bring apple cider vinegar, water, granulated sugar, and kosher salt to a boil in a small saucepan over high, stirring to dissolve sugar and salt. Remove from heat. Stir in sliced garlic cloves, dill sprigs, and crushed red pepper; and let cool for 20 minutes. Place fresh sugar snap peas, stems and strings removed, in an airtight container; pour vinegar mixture over peas. Cover and chill for 1 day.
Combine first five ingredients in a large, high-sided skillet; bring to a boil over medium-high. Reduce heat to medium; cover and simmer 10 minutes. Add salmon in a single layer; cook, uncovered, until opaque or to desired degree of doneness, about 7 minutes. Using a slotted spoon, transfer fish and onions to a plate; discard dill and liquid. Break fish into large pieces.
Place potatoes and 2 teaspoons of the salt in a medium saucepan. Add water to cover, and bring to a boil over high. Reduce heat to medium, and simmer until tender, about 10 minutes. Drain and cool 5 minutes.
Process basil, buttermilk, mayonnaise, sour cream, shallot, vinegar, pepper, and remaining 1 teaspoon salt in a blender or food processor until smooth. Combine potatoes, spinach, and 3 tablespoons of the dressing in a bowl; toss to coat.
Divide potato mixture among 4 plates. Top each with flaked salmon and Pickled Snap Peas. Drizzle each salad with remaining dressing.