Prepare the dressing: In a bowl, add in the Dijon mustard, salt, black pepper, garlic, shallot, red wine vinegar, and olive oil. Whisk together.
2 tsp Dijon mustard, ¾ tsp kosher salt, ½ tsp freshly ground black pepper, 1 clove garlic, 2 small shallots, 2 tbsp red wine vinegar, ¼ cup olive oil
Assemble: Slice the tomatoes in half, then add them on top of the dressing already in the bowl. Mix everything together to coat the tomatoes in the dressing. Let the tomatoes sit for 5-10 minutes to soak in some of the dressing. Garnish with feta cheese and parsley, if desired.