Every Indian household has their version of a mint-cilantro chutney. I grew up eating this "chutney sandwich" at birthday parties, picnics and lazy lunches every summer. The combination of fresh herbs, lime, a hint of chili with sweet juicy tomatoes and cucumbers hits the spot in a way few other things can!
In a blender or food processor, blend the cilantro, mint, garlic, chili, red onion, lime juice, salt and sugar with 1/4 cup water until smooth. Adjust salt to taste, adding more water if necessary.
Slather each piece of bread with the chutney. Place the tomato and cucumber slices on one piece of bread and dust with salt, sumac and pepper. Cover with the second slice and serve.