Bring a large pot of water to a boil. Add whole zucchini and cook until almost soft, 10 to 12 minutes. Drain and allow to cool until easily handled, about 10 minutes.
Preheat oven to 175 degrees C. Grease a baking pan.
Slice cooked zucchini lengthwise, removing the top third. Scoop out the pulp and transfer to a strainer. Pat pulp dry with a paper towel and transfer to a bowl. Add Colby cheese, cottage cheese, all but 1 tablespoon Parmesan cheese, 1/4 cup breadcrumbs, butter, sour cream, garlic, Italian seasoning, chilli, salt and black pepper. Mix to combine.
Spoon mixture into zucchini boats and place on the prepared baking pan. Sprinkle tops with remaining Parmesan and breadcrumbs.
Bake in the preheated oven until heated through and cheese is melted, 15 to 20 minutes. Turn oven grill on high and continue cooking until tops are golden brown, 3 to 4 minutes more.