In a large mixing bowl, combine the olive oil, soy sauce, lemon juice, minced garlic, ground cumin, paprika, black pepper, and salt, mixing well to create the marinade.
Add the beef cubes to the marinade, ensuring they are well coated. Cover the bowl and refrigerate for at least 30 minutes to allow flavors to meld.
Preheat your grill or grill pan over medium-high heat.
Prepare the skewers by threading the marinated beef cubes alternately with bell pepper pieces, onion wedges, and zucchini slices onto each skewer.
Place the skewers on the hot grill and grill for about 10-15 minutes, turning occasionally, until the beef is browned and cooked to your desired level of doneness.
Remove the skewers from the grill and let them rest for a few minutes. Garnish with fresh parsley if desired.