Step 1

Preheat broiler with oven rack about 4 in. from heat.

Step 2

Snap off bottom of an asparagus spear to see where it breaks easily. Line up remaining spears and slice off bottoms at the same spot. Put trimmed asparagus on a 12- by 17-in. rimmed baking sheet, sprinkle with oil and 1/4 tsp. each salt and pepper, and toss to coat. Arrange asparagus in a single layer on baking sheet.

Step 3

Season salmon with remaining 1/2 tsp. salt and 1/4 tsp. pepper, then set on top of asparagus in a single layer.

Step 4

Broil salmon and asparagus until salmon is no longer translucent but still moist in center and asparagus is browned in spots, 8 to 12 minutes. If asparagus isn't done, lift salmon to a platter and return asparagus to oven for a few more minutes.

Step 5

Meanwhile, whisk together crème fraîche, mustard, chives, and salt and pepper to taste in a small bowl.

Step 6

Serve salmon and asparagus with crème fraîche sauce.