Slice off the top and bottom of each tomato. Remove the pulp with a spoon to create hollow cylinders; set aside.
Trim the stem ends of the green beans. Quarter horizontally; cut so that the green beans are an equal length, approximately four inches. Steam for 3 to 5 minutes or just until tender; do not overcook. Place 12 green beans or as many as will fit, in each tomato; arrange the tomatoes on a serving plate.
Combine the olive oil, vinegar, mustard, salt and pepper in a bowl or blender container; whisk or process until smooth. Drizzle 1 teaspoon of the vinaigrette over each tomato. Yield: 4 servings.