Preheat the oven to 375 degrees.
Line a 9-inch pie plate with your choice of pie dough* and set aside.
*For the dough: you can make your own pastry, use a premade crust, or make it crustless by skipping dough altogether and greasing a 9-inch dish with unsalted butter to create more of an eggy meal.
*For a crispier crust: preheat the oven to 350 degrees and bake for 5-10 minutes until barely set. If crust edges are becoming too brown after filling is added, cover edges with aluminum foil during the final baking process.
In a large bowl, toss the brussels sprouts with 1/2 to 1 tbsp olive oil. Spread out evenly onto a baking sheet and roast in the oven at 375 degrees for about 20 minutes, tossing once, until browned and tender. Let cool, then coarsely chop. Reduce the oven temperature to 325 degrees when brussels are done.
While brussels are roasting, melt the butter in a large sauté pan over medium-low heat. Add the shallots and cook, stirring occasionally until soft and translucent, for just a few minutes. Do not brown.
When shallots have cooked for a few minutes, add asparagus and broccoli and mix together in the pan for a couple of minutes. Begin adding spinach by the handful, so it begins to wilt, continuing to mix veggies together.
Increase the stovetop heat to medium and add Kettle & Fire Vegetable Cooking Broth. Add salt and pepper to taste. Cook until heartier veggies (asparagus and broccoli) are tender or al dente and spinach is wilted, approximately 3-4 minutes. Strain vegetable mixture into a bowl and set aside. (You may have to press firmly with the back of a spoon to squeeze excess liquid from spinach through the strainer.) Add roasted brussels to the bowl of veggies.
In another mixing bowl, whisk together eggs, milk, and chopped green onion. Season with salt, pepper, nutmeg, and cayenne.
Spoon vegetable mixture onto pie crust, then add about half to 3/4 of the Gruyère cheese on top. Pour the egg/milk mixture over the vegetables and cheese. Sprinkle the remaining cheese on top.
Bake the quiche for 45 minutes to 1 hour (oven times vary) or until the quiche is set and slightly golden brown. Let the quiche stand for 15 minutes before cutting into wedges and serving.