1. Preheat oven to 400°F.
2. Combine 2 Tbsp extra-virgin olive oil, 1 Tbsp white wine vinegar, 1 tsp dried dill, 1 tsp spicy brown mustard, and 1 tsp chopped garlic in a blender; purée. Season with salt and pepper. Set aside.
3. Place 12 bread slices on a baking sheet; top 6 with ham, tomato, and cheese slices. Bake 5 minutes to slightly melt cheese and warm ham.
4. Pour dressing into a medium bowl; add mesclun greens and toss to coat evenly. Arrange sandwiches on 6 plates. Pile greens on each. Sprinkle with chopped olives and top with remaining slices of bread.