In a small bowl, whisk the dressing ingredients (olive oil, balsamic vinegar, and Dijon mustard) with a small hand whisk until they emulsify into a smooth salad dressing. Set aside.
Slice the tomatoes thinly. Aim to get a total of 10 slices.
Using a sharp knife, slice the mozzarella thinly into 9 rounds.
Arrange the tomato and mozzarella slices on a platter, alternating and overlapping each other. Arrange the basil leaves on top.
Sprinkle the salad with salt and pepper. Drizzle with the salad dressing. Serve immediately.
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Notes
Turn this salad into a filling main-course salad by adding avocado slices and sliced hard-boiled eggs.
Use good tomatoes. Big, red, ripe but firm, not mealy, and definitely not refrigerated. If your tomatoes did spend some time in the fridge, take them out and bring them to room temperature.
Don't store the basil leaves in the fridge - they will wilt. Place them, like flowers, in a jar with a small amount of water, away from direct sunlight.
This salad is best enjoyed fresh. The tomatoes and basil won't do well in the fridge. I recommend only making as much as you can eat immediately.