Prepare the rice according to package directions, omitting fat.
Combine the rice, tuna, artichokes, and mushrooms in a large bowl; toss gently. Combine green onions, vinegar, oil, mustard, and pepper in a bowl; stir with a whisk until blended. Add dressing to tuna mixture, tossing gently. Serve salad in lettuce-lined dishes; top with tomatoes.