Adjust rack to top position and preheat oven to 450 degrees. Wash and dry produce. • Quarter lemon. Halve potatoes lengthwise. Halve, peel, and finely chop shallot until you have ¼ cup (½ cup for 4 servings). Halve orange; squeeze juice into a small bowl until you have 2 TBSP (4 TBSP for 4). Peel and dice remaining orange into ½-inch pieces. (TIP: Remove as much of the white pith as possible to reduce bitterness.) Halve, pit, and peel avocado; dice into ½-inch pieces. Squeeze juice from one lemon wedge over avocado and sprinkle with salt. (This will keep your avocado from turning brown!) Finely chop chives.
Toss potatoes on a baking sheet with a drizzle of oil, salt, and pepper. Roast on top rack until browned and tender, 20-25 minutes. • While potatoes roast, in a small pot, combine chopped shallot, juice from one lemon wedge, 2 tsp vinegar, 1⁄3 cup water, and a pinch of sugar. (For 4 servings, use juice from two lemon wedges, 4 tsp vinegar, and ²⁄3 cup water.) Bring to a simmer, then reduce heat to medium. Cook until about 2 TBSP liquid remains, 4-6 minutes. (For 4, simmer until about 3 TBSP liquid remains, 5-8 minutes.)
While shallot mixture simmers, cut 2 TBSP cold butter into four pieces (4 TBSP into eight pieces for 4 servings). • Once liquid has reduced in shallot mixture, reduce heat to low. Whisk in butter one piece at a time, melting each piece before adding the next, until sauce is pale and creamy. • Remove pot from heat. Whisk in crème fraîche until combined. (TIP: If sauce seems too thick, add water 1 tsp at a time.) Season with salt (we used ½ tsp; 1 tsp for 4) and pepper. Cover to keep warm.
Pat salmon* dry with paper towels and season all over with salt and pepper. • Heat a drizzle of oil in a large pan over medium-high heat. Add salmon to pan, skin sides down. Cook until skin is crisp and fish is almost cooked through, 5-7 minutes. • Flip and cook until cooked through, 1-2 minutes more. Remove pan from heat.
Whisk ½ tsp vinegar (1 tsp for 4 servings) and 2 TBSP olive oil (4 TBSP for 4) into bowl with orange juice. Season with salt and pepper. TIP: Add a squeeze of lemon juice if you like a tangier dressing! • Combine mixed greens, avocado, and diced orange in a medium bowl (use a large bowl for 4). Toss with as much dressing as you like. Season with salt and pepper.
Reheat sauce over low heat if needed. TIP: If sauce has thickened, add more water 1 tsp at a time. • Divide salmon, potatoes, and salad between plates. Spoon sauce over salmon. Top salad with walnuts. Garnish salmon and potatoes with chives. Serve.