For the sauce: Grate the cucumber over a strainer and squeeze out the water. Transfer the grated cucumber to a medium bowl along with the mint, yogurt, lemon zest, lemon juice, olive oil and salt. Mix to combine, using a rubber spatula. Cover and refrigerate until ready to serve.
For the kebobs: In a medium bowl, whisk together the yogurt, olive oil, lemon zest, salt, cumin, garam masala and cayenne. Add the cubed sirloin and mix to coat evenly. Cover with plastic wrap and marinate at room temperature for about 20 minutes.
Preheat a grill or grill pan to medium heat.
Beginning with the meat, thread one cube of sirloin onto a skewer, followed by a tomato, another piece of meat, a piece of onion, another piece of meat and finishing with a tomato and an onion. Continue like this until all the skewers are assembled. Sprinkle the kebobs on all sides with salt, then place on the preheated grill and cook for three minutes per side for medium doneness. Serve warm with the cucumber-yogurt sauce.