Preheat oven to 400°F.
In a medium bowl, combine all chimichurri ingredients. Taste and adjust seasoning, if necessary, and set aside.
Season flank steak with salt, pepper, and 1/4 cup of the chimichurri sauce. Toss to coat evenly. Refrigerate for 1–2 hours.
On a large sheet pan lined with parchment, arrange squash, mushrooms, and onions in a single layer. Drizzle with a little olive oil and season with salt and pepper. Toss to coat evenly. Roast for about 15 minutes, or until vegetables start to soften and brown slightly.
After the vegetables have cooked for 15 minutes, remove the sheet pan from the oven. Add sliced flank steak to the pan, spreading it evenly over the vegetables. Return the sheet pan to the oven and cook for an additional 10–12 minutes, or until the steak is cooked through and reaches desired level of doneness. Give the meat and veggies a quick char by placing them under the broiler for 2–3 minutes.
Remove the sheet pan from the oven and drizzle with remaining chimichurri sauce. Cover with foil and let sit for 10 minutes. Serve.
Store any leftovers in an airtight container in the refrigerator for up to 4 days. To reheat, microwave for 60 seconds or warm in a skillet over medium heat.