Cook the bacon in a skillet until crisp and brown; drain on absorbent paper.

Slice off the top of the boule, and hollow out the center, leaving about a 1 inch shell. Process the soft bread in a blender for crumbs for another use. Spread the inside of the lid of the bread with butter creamed with the garlic.

Whisk the eggs in a bowl with the mayonnaise; add the spinach, bacon, nutmeg, salt and pepper to taste.

Arrange half the oysters on the bottom of the boule, cover with half the spinach mixture, half the tomato slices, a layer of crumbs, and half the cheese. Repeat the layers, and replace the bread lid. Wrap the loaf in foil and bake for 45 minutes. Cut into 6 wedges with a serrated bread knife, and serve with a toss of watercress (optional).