In a bowl, combine vegetable oil, cumin, paprika, coriander, garlic powder, onion powder, black pepper, salt, and cayenne pepper (if using).
Add lemon juice and mix well.
Add the meat cubes to the marinade and toss to coat evenly.
Cover and refrigerate for at least 1 hour, or overnight for best results.
Thread the marinated meat cubes onto the skewers, alternating with onion and bell pepper chunks.
Preheat a grill or grill pan to medium-high heat.
Place the skewers on the grill and cook, turning occasionally, until the meat is browned and cooked to your desired level of doneness, about 10-15 minutes.
Garnish with fresh parsley or cilantro if desired.
Enjoy your meal!