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