Place a small skillet over medium heat with 1 turn of the pan of EVOO, about 1 tablespoon. Add the chopped bacon to the skillet and cook until golden and crispy, about 5 minutes. Remove the bacon to a paper towel-lined plate and let cool.
While the bacon is cooking, whisk together the buttermilk, crème fraiche, garlic, dill, chives, lemon juice, horseradish, salt and pepper in a medium-size mixing bowl. Set dressing aside.
Slice each of the tomatoes in half widthwise. Lay a few rings of red onion onto the base of each tomato and then cap each tomato off with their tops.
Transfer the tomato stacks to a serving plate, top with a little of the prepared dressing and garnish with the bacon.