Get the Oven Ready: Set your oven to 375°F (190°C) to warm it up. Grease a 9-inch pie dish or a round baking dish with 1 tablespoon of ghee or avocado oil to prevent sticking.
Sauté the Vegetables: In a large skillet, heat the remaining 1 tablespoon of ghee or avocado oil over medium heat. Incorporate the diced onion and cook for roughly 3 minutes until it starts to soften. Then, stir in the minced garlic and sauté for an additional minute until it becomes aromatic.
Cook the Zucchini and Spinach: Add the sliced zucchinis to the skillet and cook for about 5 minutes, stirring occasionally, until the zucchinis are tender but still slightly crisp. Add the baby spinach to the skillet and cook for 1-2 minutes, or until wilted.
Prepare the Egg Mixture: In a large bowl, whisk together the eggs, dairy-free milk, salt, and fresh ground pepper until well combined and slightly frothy. Stir in the minced basil.
Assemble the Quiche: Spread the sautéed vegetables evenly in the prepared pie dish. Arrange the sliced tomatoes on top of the vegetables. Pour the egg mixture over the vegetables and tomatoes, ensuring that it is evenly distributed.
Bake: Place the quiche in the preheated oven and bake for 25-30 minutes, or until the quiche is set in the middle and slightly golden on top.
Garnish and Serve: Remove the quiche from the oven and let it cool for a few minutes before slicing. Garnish with additional fresh basil if desired. Serve warm or at room temperature.