HEAT oven to 350ºF.
TOSS tomatoes with next 3 ingredients in large bowl; spread onto bottom of 12x8-inch baking dish. Bake 1 hour; cool. Chop tomatoes coarsely, reserving any juices.
BROWN sausage in large skillet on medium-high heat, stirring occasionally; drain, if necessary. Stir in spinach; cook on medium heat 2 to 3 min. or just until spinach is wilted, stirring occasionally. Stir in tomatoes, reserved juices and 2 Tbsp. Parmesan. Remove from heat.
PLACE bread, cut sides up, on baking sheet; top with half the mozzarella. Cover with sausage mixture.
BAKE 12 min.; top with remaining mozzarella and Parmesan. Bake 8 min. or until mozzarella is melted. Sprinkle with oregano.