Rinse the greens in cold water and dry them in a lettuce spinner or on paper towels.
Dab the bread slices with olive oil and place on a sheet tray. Toast them for about 10 minutes at 350°F until golden. Remove from the oven.
Make the dressing: Place the Dijon mustard in a medium-sized mixing bowl. Drizzle in the oil and vinegar alternately while mixing vigorously with a whisk. Grate the 2 red tomatoes into the dressing and mix smooth. Season with salt and pepper.
Toss the greens in a salad bowl with the dressing and add the croutons followed by the cubed heirloom tomatoes and sliced cucumbers. Garnish with fresh basil.
Serve immediately or refrigerate up to 4 hours before serving.