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.