Slice the tops off of the tomatoes and scoop out the seeds and membranes, leaving enough flesh on the sides to keep the shells firm. Set the tomatoes in a dish, cover with plastic wrap, and refrigerate for 1 hour.
Stir together the mustard and vinegar in a small bowl, and slowly drizzle in the oil, whisking constantly. Stir in the red onion, pickles, and chives, and season the dressing with salt and pepper.
Combine the bologna and cheese in a medium bowl, and pour the dressing overtop, tossing gently to coat.
Spoon the dressed bologna and cheese filling into the tomatoes. Place each tomato on a plate, and garnish with a handful of lettuce.
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