In the bowl of a food processor, combine the flour, chopped thyme, and kosher salt. Pulse once or twice to mix. Add the diced cold butter and pulse several times until the butter resembles small peas. Pour the ice water into the food processor while pulsing. Continue pulsing until the dough begins to stick together in small clumps.
Dump the dough mixture onto the counter and use your hands to press it together into a large ball. Shape into a flat disc and cover with plastic wrap. Chill for at least 1 hour.
Tart Filling
Preheat the oven to 400°F. Toss the butternut squash with 1 tbsp olive oil and season with a little kosher salt and pepper. Spread the squash out on a baking sheet and roast for 30 minutes until tender. Stir the squash a few times to allow it brown evenly.
In a wide skillet, add the butter and 1 tbsp olive oil and heat over medium heat until the butter begins to melt. Add the onions along with the kosher salt. Cook for 3 to 4 minutes over medium heat until the onions begin to soften. Then turn down the heat to low.
Cook the onions over low heat until they are very tender and a deep brown colour. Stir the onions every so often so that they don’t burn or stick to the pan. When the onions are caramelized, add the minced garlic and chopped thyme and sauté for another minute. Combine the onions with the roasted squash and toss together. Allow the filling to cool slightly.
Turn the oven down to 375°F. Line a baking sheet with parchment paper and set aside. Roll out the pastry on a lightly floured surface into a large circle, about 12 inches in diameter. Transfer to the prepared baking sheet.
Spread half the filling over the pastry, leaving a 1 inch border around the edge. Crumble half of the goat cheese over top. Top with the remaining filling and goat cheese. Sprinkle with extra fresh thyme.
Gently fold the edges of the pastry up over the filling, overlapping as you go around the tart. Brush the edge with the beaten egg. Bake the tart for 45 to 50 minutes until the pastry is browned and cooked through on the bottom.
To serve, lightly dress the baby kale. Drizzle the kale with 1 tsp each olive oil and lemon juice, season with a pinch of kosher salt and toss to coat. Transfer the tart to a serving plate and top with the baby kale. Slice into wedges and enjoy!