In a large mixing bowl, combine the beef seasoning powder, coriander powder, chili powder, cumin powder, Arabic spice powder, ground black pepper, vinegar, and salt. Add the minced garlic and tamarind paste to the mixture. Stir well until all the spices are incorporated into a smooth marinade.
Add the cubed beef to the marinade, making sure each piece is well coated. Cover the bowl with plastic wrap and let it marinate in the refrigerator for at least 2 hours, or preferably overnight.
While the beef is marinating, chop the onion, green bell pepper, red bell pepper, and tomatoes into bite-sized pieces. If you’re using green chili peppers for added spice, slice them thinly as well.
Once the beef has marinated, take the skewers and begin assembling your Beef Mishkaki. Alternate threading the beef cubes with onion, green bell pepper, red bell pepper, and tomato pieces.
Preheat your grill or grill pan over medium-high heat. If using an outdoor grill, ensure it's hot enough to create a nice char on the meat.
Place the assembled skewers on the grill. Cook for about 10-15 minutes, turning occasionally to ensure even cooking. Brush with dark mushroom soy sauce during the last few minutes of grilling.
Once cooked, remove the skewers from the grill and let them rest for a few minutes. Serve your Beef Mishkaki hot, garnished with fresh herbs or additional sliced vegetables if desired.