Prepare the Crust: Preheat your oven according to your pie crust instructions. Roll out your pie crust carefully into a 9-inch pie plate ensuring it fits snugly without stretching too much.
Roast the Butternut Squash: Dice or slice your butternut squash into even pieces and toss them with olive oil, salt, and pepper before spreading them on a baking sheet. Roast in the oven until tender and lightly browned; this should take about 20-25 minutes.
Caramelize the Onions: In a skillet over medium heat, melt butter or heat olive oil before adding thinly sliced onions along with a touch of salt. Cook slowly until they become golden brown and fragrant—this process takes about 15-20 minutes.
Mix Eggs and Dairy: In a mixing bowl, whisk together eggs with whole milk (or your chosen alternative), salt, pepper, thyme, and nutmeg until well combined creating a creamy custard base.
Assemble the Quiche: Spread roasted butternut squash evenly over the pie crust followed by caramelized onions and sprinkle shredded mozzarella or Monterrey Jack cheese on top before pouring the egg mixture over everything.
Bake Until Set: This rich mixture needs around 40 minutes in the oven at your preheated temperature until it’s puffed up and lightly golden on top. Allow it to cool slightly before slicing into wedges to serve warm!