1. Preheat the oven to 450 degrees . To make the vinaigrette, whisk together mustard, capers and brine, jalapeno, garlic, lemon juice, olive oil, salt and pepper in a small bowl.
2. Spread cauliflower in an even layer on a rimmed baking sheet. Pour half the vinaigrette over cauliflower, tossing well to combine; roast cauliflower 20 minutes. Pour remaining vinaigrette over salmon and let marinate while cauliflower cooks.
3. Remove baking sheet from oven and lower temperature to 400 degrees . Push cauliflower to edges of sheet and add salmon to center. Roast 8 to 10 minutes, 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 minutes, or until crispy and slightly browned. Serve with salmon.