Combine the ingredients for the marinade: olive oil, red wine vinegar, minced garlic, chopped parsley, red pepper flakes, and about 1/4 teaspoon black pepper. Cut the steak into 1 and 1/2 inch pieces and place in the marinade. Marinate for at least 30 minutes and preferably 1-2 hours. Do not marinade longer than 4 hours to avoid mushy meat.
If using wooden skewers, soak in water at least 30 minutes before assembling.
Cut the red onion and bell peppers into 1 and 1/4 inch pieces. Place all the pepper pieces in a large bowl and lightly drizzle with vegetable oil and season to taste with salt and pepper. Toss to coat. Lightly drizzle the onion with oil and salt but don't toss so the pieces will stay together in chunks.
To assemble kebabs layer steak and veggies onto soaked skewers. Roll up a paper towel and soak in vegetable oil. Using tongs, brush the soaked paper towel over clean grill grates. Make sure grill is fully heated (to medium high heat) before adding kebabs. Place kebabs on grill and grill kebabs 6-9 minutes or until desired doneness is reached, rotating every 2-3 minutes for even grilling. (Beef should be well seared on all sides and center of beef registers between 125-130°F (for medium rare) on an instant read thermometer, about 3 minutes per side -- they continue to cook off the grill a bit). Transfer skewers to platter and let rest for 5 minutes.