Begin by slicing the sirloin into 1 inch bite-sized cubes. Season with salt and set aside.
Make the chimichurri by combining all ingredients in a food processor or blender. Pulse together until chunky paste forms. Transfer to serving bowl and let sit so that flavors can blend. Can be made up to one day ahead and stored in an air-tight container in the fridge.
Heat grill to medium high. Assemble skewers with steak cubes, slice baby bell peppers and red onion. Grill for 1-2 minutes on each side until cubes reach desired doneness. They cook quick!
Remove from grill and serve with chimichurri sauce!