Heat oven to 350 degrees Fahrenheit.
Cook bacon over a skillet until bacon is slightly crisp. Using a slotted spoon, set aside bacon in a medium sized bowl (we'll call this the "bacon bowl").
Add leek to skillet and cook until soft, about 4 minutes, and remove and set aside in bacon bowl when finished.
Working in batches, add mushrooms to the skillet and cook until golden. Remove and add to bacon bowl.
In a separate bowl (we'll call this the "wet" bowl), whisk together the milk, cream, parmesan, herbs, eggs, and salt and pepper.
Stir in bacon mixture (it should be a little cool by now), then the bread, and let sit for 15 minutes.
Grease (with butter or cooking spray) the baking dish and transfer the mixture.
Sprinkle with the Gruyere cheese and bake in oven for about 45 minutes.
Garnish with parsley and serve.