Heat the oven to 350°F. Butter a 9-inch pie dish generously.

Cook the bacon in a large skillet over medium heat until crisp. Drain on paper towels, crumble, and leave about 1 tbsp fat in the pan.

Slice the mushrooms and cook them in the bacon fat over medium-high heat, stirring occasionally, until browned and the pan is dry, about 6 to 8 minutes. Add the spinach and stir until wilted. Press out any liquid and let cool slightly.

In a large bowl, whisk the eggs, heavy cream, salt, pepper, and nutmeg until smooth.

Scatter the bacon, mushroom mixture, and 1 cup of the Swiss cheese in the pie dish. Pour the custard over the top, then sprinkle with the remaining Swiss and the parmesan.

Bake 35 to 40 minutes, until the edges are set, the top is golden, and the center has just a slight wobble.

Rest for 10 minutes before slicing into 6 wedges.