📖 Instructions

1. First, let's tackle the onions. Peel and thinly slice 3 large yellow onions. Trust me, it's worth the effort – the thinner, the better they carmelize.
2. In a large, heavy-bottomed skillet, melt 3 tablespoons of butter over medium-low heat. Add the sliced onions, 1/2 teaspoon of salt, and 1/4 teaspoon of black pepper. The salt helps draw out the moisture, which is key for even caramelization.
3. Cook the onions, stirring occasionally, for about 45-60 minutes, or until they are deeply golden brown and very soft. Be patient; this process can't be rushed. The goal is to slowly coax out their natural sugars. Keep a close eye and don't burn them!
4. While the onions are doing their thing, prepare the sauerkraut. Drain 1 pound (about 4 cups) of sauerkraut and rinse it thoroughly under cold water. Rinsing helps mellow its intense tanginess.
5. Squeeze out as much excess moisture as possible from the sauerkraut. You can use your hands, a clean kitchen towel, or even a potato ricer for this step.
6. In a large bowl, combine the caramelized onions and the squeezed-out sauerkraut. Mix them well, allowing the flavors to meld.
7. Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
8. On a lightly floured surface, roll out 1 sheet of puff pastry (about 14 ounces) to a 12-inch square. Puff pastry is your freind in this recipe; it saves loads of time.
9. Transfer the puff pastry to the prepared baking sheet. With a sharp knife, score a 1-inch border around the edge of the pastry, being careful not to cut all the way through. This creates a raised edge when baked.
10. Spread the sauerkraut and caramelized onion mixture evenly over the pastry, within the scored border.
11. In a small bowl, whisk together 1 large egg with 1 tablespoon of milk or cream to create an egg wash. Brush the egg wash over the raised edge of the pastry.
12. Bake the tart for 20-25 minutes, or until the pastry is golden brown and puffed up. Keep a close eye, because ovens are different and things can burn.
13. Remove the tart from the oven and let it cool slightly on the baking sheet before slicing and serving. A sprinkle of fresh thyme or a drizzle of balsamic glaze takes it to the next level.