Preparation

Make the “soup”

• Melt the butter in a 12-inch skillet over medium low heat. Add the onions and shallots and cook, stirring frequently, until they’re a deep caramel color, 25 to 35 minutes.
• 莱斯e the heat to medium, add the wine, and cook until it has completely evaporated, 5 to 9 minutes. Add the beef and chicken broths and cook until the liquid is reduced by about half, 5 to 7 minutes. Add the thyme and vinegar, season to taste with salt and pepper, and cook 1 minute more. Remove from the heat and let cool. Line a 4 1/2 x 8 1/2-inch loaf pan with foil and pour the “soup” mixture into the pan. Freeze until solid, at least 4 hours but preferably overnight.

Make the croutons

• Position a rack in the center of the oven and heat the oven to 400°F. In a large bowl, toss the bread with the olive oil, garlic, thyme, 1/4 tsp. salt, and 1/8 tsp. pepper. Spread the croutons on a small baking sheet and bake, stirring occasionally, until golden brown, about 15 minutes. Let cool and then skewer 40 of the best-looking croutons with toothpicks (save the leftovers for another use).

Make the dumplings

• Working with about 10 wonton wrappers at a time, arrange them on a flat surface and brush the edges with the egg. Unmold the “soup” mixture from the loaf pan. Using a chef’s knife, cut the frozen soup lengthwise into 4 long, even strips, and then cut each strip crosswise into 10 even pieces, to make 40 pieces total, each about 3/4 inch square. Put one square in the center of each wrapper and fold up the edges, pressing to create a purse-like dumpling. Repeat to make 40 dumplings. Put the dumplings on a small baking sheet or tray and freeze until ready to fry.
• Heat the vegetable oil in a 4-quart saucepan over medium-high heat until the temperature on a candy thermometer reads 365°F or until the oil begins to ripple and bubbles immediately when the edge of one dumpling is dipped into it. Have ready a tray or plate lined with paper towels. Add as many dumplings as will fit in the pan without crowding and fry until golden brown, 2 to 4 minutes. With a slotted spoon, transfer the dumplings to the paper towels. Continue frying the remaining dumplings.
• Position a rack in the center of the oven and heat the oven to 450°F. Divide the dumplings among 8 to 10 small, shallow ovenproof gratin dishes and top with the Gruyère. Bake until the cheese is melted, 5 to 6 minutes. Stick a crouton skewer in each dumpling, sprinkle with a little salt and pepper, and serve.