Preheat your oven to 375°F. While the oven heats, prepare all your vegetables: dice the onion into 1/4-inch pieces, slice the mushrooms into 1/4-inch thick pieces, dice the red bell pepper into 1/2-inch pieces, and mince the garlic fresh. Measure out the milk, cheese, and spices into separate small bowls. This prep work ensures everything is ready when you need it and prevents scrambling during cooking.

Heat the avocado oil in a large skillet over medium-high heat until shimmering. Add the diced onions and bell peppers, cooking for 2-3 minutes, stirring occasionally, until they begin to soften. Add the sliced mushrooms and minced garlic, cooking for another 5 minutes, stirring frequently, until the mushrooms release their moisture and start to brown slightly—this develops deeper flavor. Remove the pan from heat, add the baby spinach, and stir for about 1 minute until the spinach wilts. If the vegetables release excess liquid (especially from the mushrooms), drain it off using a fine-mesh strainer or by tilting the pan and pressing gently with a spoon.

In a medium bowl, beat the room-temperature eggs vigorously for about 1 minute until the yolks and whites are well combined and slightly frothy. Add the milk, salt, black pepper, and freshly ground nutmeg, then beat for another 2-3 minutes until the mixture is fully incorporated and smooth. I always take time to beat the eggs and milk thoroughly because it creates a lighter, more evenly set custard with better texture throughout the quiche.

Place the pie crust in a 9-inch pie dish, adjusting it so it sits evenly on the bottom and sides. Distribute the sautéed vegetables from Step 2 evenly across the crust, breaking up any clumps so they spread in a single layer. Sprinkle the grated cheese over the vegetables, then slowly pour the egg custard mixture from Step 3 over the entire surface, allowing it to flow around and through the vegetables. The custard should reach just below the rim of the crust.

Place the assembled quiche in the preheated 375°F oven and bake for 35-40 minutes, until the custard is set but still slightly jiggly in the very center when you gently shake the pan—it will continue to cook slightly as it cools. The top should be lightly golden. Remove the quiche from the oven and let it rest at room temperature for 5-10 minutes before slicing. I find that letting the quiche rest helps it hold together better when you cut it and gives the flavors time to settle.