For Fish: Place fish in glass baking dish. Combine mustard and lime juice in small bowl. Gradually whisk in oil. Add shallots, rosemary and generous amount of pepper. Spoon marinade over fish, turning to coat. Let stand at room temperature 30 minutes or chill 3 hours. For vegetables: Cook potatoes in large pot of boiling water until just tender. Drain. Cool. Slice potatoes. (Can be prepared 3 hours ahead. Cover and chill.) Prepare barbecue (medium-high heat) or preheat broiler. Sprinkle fish with salt. Without removing marinade, grill fish until just cooked through, about 4 minutes per side. Place fish on platter. Meanwhile, heat oil in large skillet over medium heat. Add shallots and rosemary; cook until shallots are tender, about 5 minutes. Add all squash; saut until crisp-tender, about 8 minutes. Add potatoes and tomatoes; stir until heated through. Season with salt and pepper. Spoon vegetables around fish. Garnish with rosemary sprigs, if desired. Serves 4. Bon Appetit August 1993