Melt the butter in a large saute pan, add the spinach and cook, stirring until completely wilted, about 4 minutes. Add the oysters and the garlic. Cook, stirring until the oysters plump, about 5 minutes. Using a wooden spoon, fold in the Mornay sauce, and season. Cook until completely heated through and pour into the tart shell. Sprinkle the top of the tart with cheese and blast under the broiler until it melts, 3 minutes or so. Meanwhile, fry the spinach leaves in hot oil for 2 minutes, season. When you remove the tart from the oven, top with the fried spinach leaves.