In a small bowl, whisk together the extra-virgin olive oil, balsamic vinegar, and Dijon mustard to create the vinaigrette. Gently fold in the chopped oregano, parsley, and chives.
Arrange the red and yellow tomato slices in a single layer in a shallow, nonreactive dish (like glass or ceramic).
Pour most of the herb vinaigrette over them, reserving about 2 tablespoons (30 ml / 1 fl oz) for the bread.
Let the tomatoes marinate at room temperature for at least 10 minutes to absorb the flavors.
Toast the slices of whole-grain bread until golden and crisp.
Arrange the toasts on a platter and drizzle with the reserved 2 tablespoons (30 ml / 1 fl oz) of vinaigrette.
Artfully arrange the marinated tomato slices on the toasts, alternating between the red and yellow colors.
Season generously with coarse salt and freshly ground black pepper just before serving.
Add a clove of finely minced garlic to the vinaigrette for an extra layer of savory flavor.