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