Mix together marinate ingredients in a medium bowl. Cut sirloin steak into 1/2 inch strips and place into the bowl with marinate. Place into refrigerator, make chimichurri and prep the veggies in the meantime.
For the chimichurri: In a food processor, add shallot, garlic, and red wine vinegar. Pulse until roughly chopped. Stop and scrape down the sides. Add oregano, parsley, and basil. Slowly add olive oil while pulsing. Stop and scrape down the sides once more. Stop pulsing when herbs are finely chopped. Stir in salt, pepper, and red pepper flakes.
Prep the veggies: Cut off ends of onion and cut into quarters, place into large bowl. Remove core and seeds from bell peppers, cut each bell pepper into 8 pieces, transfer to bowl. Slice summer squash into 1/2 inch rounds, transfer to bowl. Add grape tomatoes and HALF of the chimichurri sauce to the bowl, toss to combine.
Assemble skewers: Thread steak (accordion-style) and veggies onto 10-inch skewers in desired order. 2 steak pieces per skewer is recommended.
Grill: Preheat grill to medium high. Once hot, place skewers on the grill. Grill 6-8 minutes per side or until desired doneness, turning occasionally. Transfer to serving dish. Drizzle with remaining chimichurri sauce before serving.