Cut salami into 2-inch pieces. Cut pieces into 1/4-inch thick sticks. Put into mixing bowl.
Trim tips and ends of green beans. Cut into pieces about 3-inches long. Steam for 10 minutes or until beans are tender. Drain. Combine beans with the salami in a bowl. Add the green onion.
Cut tomatoes into wedges. Add to vegetables and salami.
For the dressing, put mustard and vinegar into small mixing bowl. Add salt and pepper. Slowly whisk in the olive oil until mixture is creamy. Pour dressing over salami and vegetables. Turn over gently with spatula until all pieces are coated. Cover and refrigerate for 2 hours or more.