Preheat the oven to 375°F (190°C) and grease a 9-inch pie dish or similar baking dish.

Heat olive oil in a skillet over medium heat until shimmering and warm.

Sauté the diced onion until translucent, stirring to prevent browning beyond color.

Add minced garlic and cook briefly until fragrant, about thirty seconds to one minute.

Stir in sliced mushrooms and cook until they release moisture and soften evenly.

Add chopped spinach and continue cooking until fully wilted and excess moisture has mostly evaporated.

Whisk together the eggs and milk in a mixing bowl until the mixture is uniformly pale and slightly frothy.

Fold in crumbled feta, then add salt and pepper sparingly because feta already contributes salt.

Stir the sautéed vegetable mixture into the egg-feta mixture until evenly distributed without overmixing.

Pour the combined mixture into the prepared greased pie dish, spreading ingredients evenly across the surface.

Bake in the preheated oven for twenty-five to thirty minutes, or until the center is set and the top shows light golden color.

Let the quiche cool for a few minutes on a wire rack before slicing to finish setting the custard.