HEAT oven to 400ºF.
CUT zucchini lengthwise in half; scoop out centers onto cutting board, leaving 1/8-inch-thick shells. Place shells in 13x9-inch baking dish.
CHOP zucchini flesh. Melt 1 Tbsp. butter in large skillet on medium-high heat. Add onions and chopped zucchini; cook and stir 3 min. or until onions are crisp-tender. Add tomatoes and bacon; cook 2 min. Stir in cheese. Spoon into zucchini shells.
MELT remaining butter in same skillet. Add panko crumbs; cook and stir 3 min. or until golden brown. Sprinkle over filled shells.
BAKE 15 min. or until heated through.