Matzo Balls with Caramelized Onions

Ingredients:

• 4 tablespoons unsalted butter
• 2 large onions, thinly sliced
• 1/4 tbsp. dry sherry
• 4 large eggs, lightly beaten
• 1/4 cup seltzer water
• 1/4 cup schmaltz (rendered chicken fat), room temperature
• 1.5 cups matzo flour
• 170 g Gruyere, cut into 12 cubes
• Your favorite chicken soup, for serving (optional)

Preparation:

1. In a large skillet, melt the butter over medium-low heat. Add the onion and cook, undisturbed, until softened, about 20 minutes. Continue cooking, stirring every 5 to 10 minutes, until the onion is golden brown and caramelized, about 1 hour more. Increase the heat to medium; add the sherry and cook, stirring, until evaporated, about 2 minutes. Transfer the onion to a plate, spread it out, and let it cool in the refrigerator for 20 minutes.
2. Place 1 cup of the cooled onion in a food processor and pulse until smooth (reserve the remaining onion for another use). Transfer the onion to a large bowl; add the eggs, seltzer water, schmaltz, and 1 teaspoon of salt, and pulse with a fork until smooth. Add the matzo meal and continue to pulse with a fork until fully incorporated and the mixture resembles a thick paste. Refrigerate for 30 minutes.
3. Bring water to a boil in a large saucepan.
4. Meanwhile, wet your hands with water, divide the matzo mixture into 12 equal portions, and flatten each into a disk. Add 1 cube of cheese to the center of each disk and roll into a ball, completely enclosing the cheese.
5. Add the matzo balls to the boiling water, reduce the heat, and simmer until the balls have almost doubled in size and are light and fluffy, about 30 minutes. Remove them with a slotted spoon and add them to your favorite chicken soup.