📖 Instructions

1. **Prepare the Pastry Dough (approx. 15 minutes active, 30 minutes chilling):** In a large bowl, combine the all-purpose flour and salt. If using a food processor, pulse flour and salt together. Add the cold, unsalted butter, cut into small cubes. Using your fingertips or a pastry blender (or pulsing in the food processor), cut the butter into the flour until the mixture resembles coarse meal with some pea-sized butter chunks still visible. This texture is key for flakiness, so resist overmixing! Slowly add the ice water, one tablespoon at a time, mixing just until the dough comes together. Be careful not to add too much water; the dough should be shaggy but hold together when pressed. Form the dough into a disc, wrap it tightly in plastic wrap, and chill in the refrigerator for at least 30 minutes.
2. **Caramelize the Onions (approx. 45-60 minutes):** While the dough chills, peel and thinly slice your yellow onions. Heat the olive oil and a tablespoon of butter in a large, heavy-bottomed skillet or Dutch oven over medium-low heat. Add the sliced onions and a pinch of salt. Stir well to coat. Cook the onions slowly, stirring occasionally, for 45-60 minutes, or until they are deeply golden brown, very soft, and sweet. If they start to brown too quickly, reduce the heat. You might need to add a tablespoon of water if they stick. Towards the end, stir in the brown sugar (or balsamic vinegar) for an extra layer of sweetness and depth, then add fresh thyme leaves. Remove from heat and let cool slightly.
3. **Preheat Oven and Prepare Molds (approx. 5 minutes):** Preheat your oven to 375°F (190°C). Lightly grease a mini muffin tin or tartlet molds. If you don't have mini tartlet molds, a regular muffin tin will work for slightly larger tartlets.
4. **Roll and Cut Dough (approx. 10 minutes):** On a lightly floured surface, roll out the chilled pastry dough to about 1/8-inch thickness. Use a round cookie cutter (about 2.5-3 inches in diameter, depending on your molds) to cut out circles. You should get about 12-18 circles. Re-roll scraps gently if needed.
5. **Assemble the Tartlets (approx. 10 minutes):** Gently press each dough circle into the prepared mini muffin or tartlet molds, making sure the dough goes up the sides. Fill each pastry shell with a spoonful of the cooled caramelized onions. Top the onions with a sprinkle of crumbled goat cheese or shredded Gruyère cheese.
6. **Bake the Tartlets (approx. 20-25 minutes):** Place the filled tartlet molds on a baking sheet (to catch any potential drips). Bake for 20-25 minutes, or until the pastry is golden brown and the cheese is bubbly and slightly browned. Keep an eye on them, as oven temperatures can vary!
7. **Cool and Serve (approx. 5 minutes):** Carefully remove the tartlets from the oven. Let them cool in the molds for a few minutes before gently transferring them to a wire rack to cool completely. They’re delicious warm or at room temperature. Garnish with a few extra fresh thyme sprigs if you're feeling fancy. Enjoy every savory, sweet bite!