1/4 cup mayonnaise
1 teaspoon wasabi paste
1 1-inch piece ginger, peeled, finely grated
2 large garlic cloves, finely grated
4 6-ounce skinless salmon fillets
Kosher salt, freshly ground pepper
1 pound baby bok choy, halved
2 cups (packed) finely shredded green cabbage (about 5 ounces)
4 ounces shiitake mushrooms, stemmed, sliced if large
2 tablespoons olive oil
Preheat oven to 450°. Heat a large rimmed baking sheet for 15 minutes.
Meanwhile, mix mayonnaise and wasabi in a small bowl. Stir in half of ginger and half of garlic; set aside. Season fish all over with salt and pepper. Place bok choy, cabbage, and mushrooms in a large bowl. Drizzle with oil and add remaining ginger and garlic. Toss to coat; season with salt and pepper.
Scatter vegetables across one side of baking sheet. Arrange salmon on other side. Roast, stirring vegetables occasionally, until salmon is cooked through, 12–15 minutes. Divide vegetables among plates; top with salmon. Serve wasabi mayonnaise alongside.