Place the vinegar, oil, sugar, salt, and a few grinds of pepper in a large bowl and whisk to combine.
Slice the cucumbers into thin 1/8-inch-thick rounds. Place them in the bowl, add the chives, and toss to combine. Cover and refrigerate for at least 30 minutes or overnight to allow the flavors to meld. Taste and season with salt and pepper as needed before serving.
Preheat the oven to 450°F.
1. Whisk together the first 6 ingredients in a large bowl.
2. Spread a rimmed (18×13-inch) half sheet pan with the tablespoon of olive oil.
3. Dunk the salmon in the miso glaze and arrange along one edge of the baking sheet.
4. Next, toss the asparagus in the glaze and arrange in one even layer in another corner of the baking sheet.
5. Finally, toss the bok choy with the remaining glaze and arrange in a pile in the open space on the baking sheet (don’t worry about piling it up a little—it will help them steam and cook through).
6. Roast in the oven for 12 to 15 minutes, or until the fish is firm to the touch (if you’d like a little more color on the salmon, broil it for 2 minutes at this point).
7. Remove from the oven, garnish with sliced scallions and sesame seeds, and serve.