At Nonette, a bustling bánh mì and doughnut shop in Paris, we enjoyed a fantastic Vietnamese sandwich filled with runny-yolked fried eggs plus the traditional bánh mì toppings of tangy, quick-pickled vegetables, thinly sliced cucumber and fresh cilantro. For added depth and umami, chef Khánh-Ly Huynh seasoned the eggs with Maggi, a liquid flavor enhancer commonly used in Vietnamese cooking. Maggi is sold in most supermarkets; look for its distinctive dark-brown shaker bottle with a red and yellow label. Soy sauce is a suitable alternative; just use about ½ teaspoon per sandwich in place of the Maggi. The best type of bread for these sandwiches is a supermarket-style baguette with an airy crumb and thin, brittle crust. Avoid chewy, rustic bread. You'll need four 7- to 8-inch pieces, so cut one or two baguettes into sections, as needed.