Preheat oven to 400°F (200°C). In bowl, toss together shallots, carrots, radishes, olive oil and 1/4 tsp (1 mL) salt. Arrange in single layer on large baking sheet. Roast in oven, turning occasionally, 25 min., or until vegetables are lightly golden and tender. Set aside.
Meanwhile (after fish is cooked), use melted butter and juices from Seafood Steamer bags to make warm vinaigrette by whisking the liquids from bags with vinegar, maple syrup and grainy mustard.
Gently toss arugula with roasted vegetables. Transfer to platter. Break salmon into large pieces and scatter over salad. Sprinkle with walnuts. Drizzle with warm vinaigrette.