Preheat the oven to 375°F., To prepare the crust: Roll the prepared crust into a 12" circle (to fit a 9" pie pan). Place it in the pan and prick it all over with a fork. Bake the crust for 10 minutes, then remove it from the oven and set it aside to cool., To make the filling: Quarter the onion, and slice thinly. Heat 1 tablespoon of the olive oil in a medium saucepan over medium-low heat. Add the onion and cook, stirring occasionally, until caramelized and deep golden brown. This will take about 20 minutes. Right before the onion finishes cooking, add the balsamic vinegar to the pan and cook for a few more minutes. , While the onion is caramelizing, spread the cubed butternut squash on a parchment-lined baking sheet and toss it with the remaining olive oil, along with a couple of good dashes of salt and pepper. Roast the squash until it starts to brown and soften; this should take about the same amount of time as the onions., In a large bowl, whisk together the milk and eggs. Add the rosemary, thyme, salt, and pepper. Add the caramelized onions, roasted squash, grated cheese, and baby spinach. Pour the mixture into the pie crust. Don't overfill! If you find you have too much filling, just leave a little out, baking it alongside the pie in a custard cup, if desired., Bake the quiche for about 40 to 45 minutes. The edges should be golden brown and the center should feel just set. , Remove the quiche from the oven and let it cool on a rack. Serve warm, or at room temperature. Refrigerate any leftovers.