Preheat oven to 350 degrees. Place the pie shell or dish on top of a baking sheet, or if you prefer to go crustless, use a glass pie dish and spray with non-stick oil.
Melt butter in a large saucepan over medium heat. Add shallots and cook for 2-3 minutes. Then add garlic, mushrooms, a sprinkle of salt and pepper, and 1 Tbsp of water. Continue to cook another 5-8 min, or until mushrooms are soft, mixing often.
Meanwhile, whisk eggs, half-and-half, salt, and pepper together. Set aside.
Once the mushrooms are soft, lower the heat and add the spinach (about 2 handfuls) and fold in gently. Remove from heat as soon as the spinach wilts.
Spoon the mushroom/spinach mixture (juices too) into the bottom of the pie shell or dish, making sure it's evenly distributed. Then pour the egg mixture over the top. Lastly, add in the goat cheese. Use your fingertips to gently push the spinach leaves and cheese down into the egg mixture. Carefully place the baking sheet onto the middle rack and cook for 40 min or until set. Let rest for 10-15 minutes before serving.