Place the potatoes in a pot. Cover by 1-2 inches in cold water. Season with 2 teaspoons kosher salt. Bring to a boil, then boil the potatoes for 5-7 minutes, or just until fork-tender. Drain & set aside.
Meanwhile, as the potatoes par-boil, prep the chimichurri sauce. Add all listed ingredients to a food processor. Pulse to combine. Taste and season with additional S&P. Set aside.
Thread each skewer with potato, top sirloin, bell pepper, red onion, alternating as you go. Pour ½ of the prepared chimichurri over the steak kabobs, using your hands to ensure each is nicely coated in chimichurri sauce. Reserve remaining chimichurri for serving.
Prepare a grill for medium-high direct heat grilling (500-600 degrees F).
Once hot, place the steak kabobs on the grill, such that the kabobs run perpendicular to the grill grates. Grill 3 minutes per side for medium doneness – add or subtract a minute of cooking time per side if you prefer your steak more/less done.