Heat broiler. Combine 2 tablespoons mustard, oil, 1/4 teaspoon salt and 1/8 teaspoon pepper in plastic food-storage bag. Add pork; seal; turn to coat. Marinate at room temperature for 15 minutes.
Broil pork 5 inches from heat 30 minutes, turning every 10 minutes, until golden and instant-read thermometer registers 160 degrees F.
Meanwhile, saute onion and mushrooms in melted butter in skillet 6 minutes. Remove from heat. Stir in flour until blended. Stir in remaining 3 tablespoons mustard and the milk until smooth. Return to heat; simmer, stirring, 2 minutes. Add broccoli, remaining salt and pepper; cook 2 to 3 minutes or until heated through.
Let tenderloin stand 5 minutes. Slice and serve with mustard sauce. Makes 6 servings.