Preheat oven to 400° F.
Fry bacon in 12-inch skillet until crispy, but not over cooked. I prefer cast iron here. Remove cooked bacon from skillet and set aside leaving bacon grease in pan.
Brown sausage in empty skillet with bacon grease. When browned remove from skillet and set aside.
Add 2 tbsp butter to pan. When foaming add leeks, celery and 1/2 tsp salt and sauté until soft and starting to brown. Remove from pan and set aside. This image is from an earlier test on the recipe when quantities were smaller. Sauting the vegetables separately allows for better liquid reduction.
Add remaining 2 tbsp butter to empty skillet. When foaming subsides add chopped mushrooms, onions and 1/2 tsp salt. Cook over medium high heat until browned. Add beech mushrooms and cook slightly.
Return leeks, celery and sausage to pan and reheat. Add spinach in handfuls stir in until wilted. Season with salt and pepper to taste. Remove pan from heat.
Remove oysters from liquid--cut larger oysters in half--add to pan. Add bacon to pan, stir to combine. At this point you can transfer mixture to 9 x 13 baking dish or leave in pan. Top with parmesan cheese and then top with gruyere. Place in 400° F oven and bake until cheese is lightly browned--about 20 minutes.