2 tablespoons red-wine vinegar
1 teaspoon Dijon mustard
1 teaspoon salt
1/2 teaspoon sugar
1/2 teaspoon black pepper
1/2 cup extra-virgin olive oil
4 lb mixed heirloom tomatoes, quartered if small or cut into 1/4-inch-wide wedges if larger
1 lb cherry tomatoes
1 lb very small mozzarella balls (1/4 inch; sometimes called perlini) or 1 lb lightly salted mozzarella, cut into 1/4-inch pieces
1 1/2 cups loosely packed small basil leaves or torn large leaves
Whisk together vinegar, mustard, salt, sugar, and pepper in a large bowl.
Add oil in a slow stream, whisking constantly until dressing is emulsified.
Add tomatoes, mozzarella, and basil and toss well.
Season with salt and pepper.
Salad, without vinaigrette, can be assembled 2 hours ahead and kept at room temperature. Salad can be dressed 1 hour ahead and kept at room temperature, covered.
Vinaigrette keeps, covered and chilled, 1 week.