In a skillet over medium heat, toast cumin, coriander and peppercorns until fragrant, 2 minutes. Using a spice grinder or coffee grinder, grind the spices. In a medium bowl, combine the spice mixture, yogurt, onion, tomato, chilies and salt. Finely chop mint and cilantro then mix into the raita. Peel and finely chop the cucumber, add to the raita and stir completely.