Butter a 13×9-inch baking pan with 1 Tablespoon butter. Arrange bread cubes in the bottom of the pan. Top bread evenly with crabmeat.
Melt 1 Tablespoon butter in a large skillet over medium-high heat. Add onion; sauté for 4 minutes. Add spinach; cook 2 minutes or until spinach wilts.
Arrange spinach mixture over the crabmeat. Top with grated Cheddar and Parmesan cheeses.
Melt ¼ cup butter. Combine milk, beaten eggs, ¼ cup melted butter (4 tablespoons), dry mustard, salt, pepper and thyme. Pour egg mixture evenly over the pan. Cover with foil. Chill in the refrigerator for 8 hours or overnight.
Bake at 325°F, covered, for one hour or until bubbly.