How to Make It
Preheat the oven to 400°. On a large rimmed baking sheet, toss the tomatoes with 1/4 cup of the olive oil and season with salt. Roast for about 20 minutes, until softened and browned in spots; let cool slightly. Transfer the tomatoes to a food processor. Add the parsley, vinegar, crushed pepper and the remaining 1/4 cup of oil. Pulse until almost smooth. Season with salt and transfer to a bowl.
In a saucepan, cover the potatoes with water and bring to a boil. Add a generous pinch of salt and simmer over moderate heat until tender, about 15 minutes. Drain the potatoes and let cool, then halve lengthwise.
In a large skillet, heat the olive oil until shimmering. Add the onion and cook over moderately high heat, stirring, until just softened, about 3 minutes. Add the potatoes cut side down and cook until browned on the bottom, about 5 minutes. Flip the potatoes and onion and cook, stirring, until well browned, about 4 minutes. Stir in the butter and parsley and season with salt. Transfer to a platter and keep warm.
Light a grill or preheat a grill pan. Season the steaks with salt and black pepper. Grill over high heat, turning once, until medium-rare, about 7 minutes total. Transfer to a carving board and let rest for 10 minutes. Thinly slice the steak across the grain and transfer to the platter with the potatoes. Serve with the tomato chimichurri.
The chimichurri can be refrigerated for up to 3 days.