The Modern Proper on Instagram: "Making vacation smash burgers with not so secret burger sauce. No better burger. Trust us . Burger Sauce 1 finely chopped grilled onions 1/2 cup mayonnaise 1/2 cup yellow mustard 1/2 cup ketchup 1 Burger 4 ounces 80/20 ground beef, rolled into a ball 1/2 teaspoon kosher salt 1 teaspoon yellow mustard 1/4 cup grilled onions 1 slice American Cheese 1 Bun Lettuce and Tomato optional In a medium bowl combine grilled onions, mayonnaise, mustard and ketchup. Set…