1 sheet puff pastry
1 tbsp extra virgin olive oil, plus more for serving
2 cups onion (from 1 lg or 2 small), ends removed, halved root to stem, peeled and sliced
1 egg yolk
1 tbsp milk
½ cups Gruyere, grated, plus more for serving
1 tbsp fresh thyme, divided
3 small or 2 large (about ½ lb) Lakeside Produce heirloom tomatoes, sliced ¼-in thick
Take out puff pastry sheet and let thaw on counter while you caramelize the onions. Heat olive oil in a large skillet over medium. Once oil is shimmering, add onion and season with salt to taste. Sauté for 10 minutes, stirring periodically, adding a bit of water if the skillet gets dry (the water will also help give the onion a nice uniform brown color). Reduce heat to medium low and continue cooking for 30 minutes, stirring every so often and adding a bit of water as needed. Once caramelized, remove from heat and set aside.
Preheat oven to 400°F. In a small bowl, whisk together egg yolk and milk for your egg wash.
Place thawed puff pastry sheet on a sheet of parchment paper. Lightly flour a rolling pin and roll into a 10x15-inch rectangle. Use a fork to prick puff pastry all over. Carefully transfer pastry and parchment onto a large sheet pan.
Spread caramelized onions on top of pastry, in an even layer, leaving a 2-inch border around all sides. Top with half of the grated gruyere and sprinkle with half of the thyme. Arrange tomato slices on top, overlapping them as needed. Season tomatoes with salt, to taste. With a pastry brush, lightly brush border with egg wash. Fold the pastry over the sides. Brush with more egg wash. (If pastry has gotten too warm, please tart in refrigerator for 10 minutes before baking.)
Place tart in oven for about 25-30 minutes, or until pastry is golden. Remove from oven, drizzle with olive oil, grate more cheese over top and sprinkle with remaining ½ tbsp thyme. Let cool a few minutes before serving. To serve, cut tart into 6 squares.