Preheat your oven to 425°F. On a baking sheet, toss the mushrooms, spinach, and leeks with olive oil and 1/2 tsp salt. Roast for 12-15 minutes, flipping halfway, until they are browned and fragrant. Transfer the roasted veggies to a large bowl to cool slightly. Clean the baking sheet and position it back in the oven on the lower third rack.

While the veggies cool, place the pie dough in a 9-inch pie dish, crimping the edges as desired. Freeze for 10-15 minutes until it firms up.

In the bowl with the roasted vegetables, add the eggs, cream or milk, cheese, and remaining salt. Whisk everything together until the mixture is frothy.

Put the chilled pie crust on the preheated baking sheet. Pour the filling into the crust and bake for 15 minutes at 425°F.

Reduce the oven temperature to 300°F and continue baking for 20-25 minutes until the center is mostly set but still slightly wobbly.

Let the quiche rest for 20 minutes at room temperature to set before slicing.