1. If you like, flute mushrooms. Set aside.2. Cut four thick center slices from tomatoes; reserve any leftover tomatoes to use in salad another day. In 10-inch skillet over medium heat, in 1 teaspoon hot butter or margarine, cook tomato slices until heated through. Remove to warm large platter.
3. Meanwhile, in 12-inch skillet over medium heat, in 2 tablespoons hot butter or margarine {1/4 stick), cook onion until tender, stirring occasionally. Remove to bowl. In drippings remaining in skillet, over medium-high heat, cook mushrooms and tenderloin steaks about 5 minutes for rare or until of desired doneness, turning steaks once. Remove steaks and mushrooms to platter, placing one steak on each tomato slice; keep warm.
4. Into drippings remaining in skillet, stir water, mustard, and capers; over medium heat, cook, stirring constantly, until mixture thickens slightly and boils. Return onions to skillet; heat through. Pour sauce over and around steaks. Garnish platter with watercress sprigs.