Preheat oven to 375°.
Arrange the baguette slices on 2 baking sheets; bake baguette slices at 375° for 8 minutes or until crisp.
Place 1/2 (1-ounce) bread slice in a food processor; pulse 10 times or until coarse crumbs measure 1/4 cup.
Drain the oysters in a colander over a bowl, reserving 2 tablespoons oyster liquid. Place the oysters on a broiler pan, and broil 7 minutes or until the edges of oysters curl. Cool and chop.
Place onions, celery, and parsley in a blender or food processor, and process until finely chopped.
Melt butter in a Dutch oven over medium-high heat. Add onion mixture; sauté 5 minutes or until tender. Add spinach; cook 2 minutes or until thoroughly heated. Stir in oysters, 1/4 cup breadcrumbs, 2 tablespoon oyster liquid, cheeses, and the remaining ingredients; cook 2 minutes or until well blended, stirring constantly.