Preheat the oven to 450°F. To make the vinaigrette, whisk together mustard,capers and brine,jalapeño,garlic,lemon juice,olive oil,salt and pepper in a small bowel.

Spread cauliflower in an even layer on a rimmed baking sheet. Pour half the vinaigrette over cauliflower, tossing well to combine; roast cauliflower 20mins. Pour remaining vinaigrette over salmon and let marinate while cauliflower cooks.

Remove baking sheet from oven and lower temp to 400°F. Push cauliflower to edges of sheet and add salmon to center. Roast 8-10 mins,or until salmon is opaque throughout. Remove baking sheet from oven, transfer salmon to a plate and turn on the broiler. Toss cauliflower again and spread in an even layer. Broil 2 to 4 mins, or until crispy and slightly browned. Serve with salmon.