Preheat the oven to 375°F (190°C). Place the store-bought pie crust into a 9-inch pie dish and prick the bottom with a fork. Pre-bake the crust for 8-10 minutes to prevent it from getting soggy.
Cook the Vegetables:
Heat a little olive oil in a pan over medium heat. Sauté the mushrooms until they release their moisture and are golden brown, about 5 minutes. Add the spinach and cook until wilted, about 1-2 minutes. Set aside.
Make the Filling:
In a large bowl, whisk together the eggs, milk (or heavy cream), salt, pepper, and dill. Stir in the drained tuna, cooked bacon, sautéed spinach, mushrooms, and shredded cheese.
Pour the tuna, spinach, bacon, and mushroom mixture into the pre-baked crust, spreading it out evenly.
Bake the quiche in the oven for 30-35 minutes, or until the center is set and the top is golden brown. If the crust edges are browning too quickly, cover them with foil.
Cool and Serve:
Let the quiche cool for 5-10 minutes before slicing. Serve warm or at room temperature.