Stuffed Mushroom Casserole

I love stuffed mushrooms, but unless we have a large party to attend, I don't feel like going through all the trouble of making a whole tray of them just so I can eat every single one.  Enter this recipe from An Edible Mosaic.  Wow, talk about the perfect remedy to that problem!  Mine doesn't look nearly as pretty as hers does, but it tasted quite delicious!  It was very easy to throw together and reheated quite nicely the next day, as well.

1 tablespoon Butter, diced
1 ounce Parmesan, grated (divided)
4 tablespoons Olive Oil
2 Celery Stalks, finely diced
1 Onion, diced
1/4 teaspoon Salt
1/8 teaspoon Pepper
1 Bay Leaf
1/2 pound Mushrooms, thinly sliced (I used a combination of white and baby bellas)
4 Garlic Cloves, minced
1 1/2 teaspoons Fresh Rosemary (I used 1/2 teaspoon dried), minced
1 1/2 teaspoons Fresh Thyme (I used 1/2 teaspoon dried), minced
1/2 cup Breadcrumbs
1 Egg, lightly beaten
1 1/2 teaspoons Worcestershire Sauce
4 ounces Cheddar, shredded (divided)

Lightly grease 4 individual sized gratin dishes or an 8x8 pan.  Transfer 2 tablespoons of grated parmesan to a small bowl and set aside.  Heat the oil in a large skillet over medium-high heat.  Add the celery and cook 3 minutes, then add the onion, salt, pepper, and bay leaf and sauté for 7 minutes.  Add the mushrooms and cook until all of the vegetables are tender and the mushrooms are browned, about 7-10 minutes.  Add the garlic, rosemary, and thyme and cook 1-2 more minutes.  Cool slightly.  In a large bowl, stir together the vegetable mixture with the breadcrumbs, egg, worcestershire sauce, the larger portion of parmesan cheese, and half of the cheddar cheese.  Divide the mix between 4 dishes or pour it into the 8x8 pan.  Sprinkle the remaining cheddar cheese and the reserved 2 tablespoons of parmesan cheese on top and dot with butter.  Bake until the cheese is melted and golden brown on top, about 10-15 minutes.  Broil for a few minutes, if desired.