Pulse all herbs, lime juice and vinegar in food processor until finely chopped.
Heat Extra Melt® Cheese Sauce, whisking occasionally, until melted and smooth. Stir herb mixture into cheese sauce. Heat to 165°F. Hold at 145°F-150°F unitl service.
Toast buns. Spread about 1 tablespoon Chimichurri sauce on cut sides of each bun. To build sandwich, layer 4 ounces roast beef and sautéed onions on botom half of bun. Top with 2 tablespoons Chimichurri sauce; cover with top half of bun.