Preheat your oven to 325 degrees. Toast the diced bread for approximately 5 minutes. Set aside to cool.
Melt the butter over medium heat in a saucepan. Once the butter is melted, add the sliced shallots and sauté until aromatic and translucent in color.
While the shallots are cooking, place the toasted bread, oysters, spinach, tarragon, lemon juice and zest in a mixing bowl. Add ½ of the Gruyère cheese (1/2 cup). Once the shallots are cooked, pour the hot butter and shallot mixture over the ingredients in the mixing bowl. Begin to work the ingredients together with a wooden spoon. They will come together much like a classic turkey stuffing.
With all the ingredients combined, place the mixture in an oven-safe casserole dish. Top with the remainder of the Gruyère cheese.
Bake the dressing for 25 minutes or until golden brown on top.
Remove from oven and let stand 5 minutes prior to serving.