Cut zucchini lengthwise in half, scoop out centers onto cutting board, leaving 1/8-inch-thick shells. Place shells in a 13x9-inch baking dish.
Chop the zucchini flesh. Melt 1 Tbsp. butter in a large skillet on medium-high heat. Add onions and chopped zucchini; cook and stir 3 minutes or until onions are crisp-tender. Add tomatoes and bacon; cook 2 minutes. Stir in cheese. Spoon into zucchini shells.
Melt remaining butter in same skillet. Add panko crumbs; cook and stir 3 minutes or until golden brown. Sprinkle over filled shells.
Bake in a preheated 400-degree oven for 15 minutes or until heated through.
Serve this as a vegetable-based side dish with a serving of lean meat or poultry. Or serve alone as an appetizer with a marinara dipping sauce.