Heat a large, heavy-bottomed skillet over medium heat. Add thinly sliced onions with a pinch of salt and cook, stirring occasionally, until deep golden brown, about 15-20 minutes. Deglaze with white wine or vegetable stock and cook for an additional 5 minutes, then set aside.

Preheat your oven to 400°F (200°C). Peel and cube the butternut squash, toss with olive oil, salt, and pepper. Roast for 25-30 minutes until tender, stirring halfway.

In a mixing bowl, whisk together eggs, milk, salt, pepper, thyme, and nutmeg. This creamy custard will be the base for your quiche.

Line a 9-inch pie dish with chilled pie crust. Sprinkle mozzarella or Monterey Jack cheese on the bottom, layer the roasted squash and caramelized onions, crumble goat cheese on top, and pour custard mixture evenly.

Bake the quiche for 30-40 minutes or until the filling is set and lightly golden brown. It's done when a toothpick inserted in the center comes out clean.

Allow to cool for 10-15 minutes before slicing. Serve warm or at room temperature.