Preheat oven to 350°F. Use a pastry brush to lightly coat the bottom and sides of a casserole dish. Set aside.
Peel the butternut squash (see my method for ease of peeling above). Slice in half lengthwise and scoop out seeds from the bulbus end of both halves. Cut into 1/2-inch thick slices, then into 1/2-inch cubes. Add to a large mixing bowl and set aside.
In a large non-stick skillet, sauté the finely chopped onion and the chopped garlic for 3 to 5 minutes over medium-high heat. You want the onion first translucent, but then to turn golden. Avoid allowing the garlic to burn by constantly moving the mixture around in the pan.
In a medium sized mixing bowl, combine the panko, grated parmesan, lemon zest, finely chopped rosemary, and finely chopped parsley and stir well to combine. Spoon enough of the garlic butter mixture into the panko mixture and stir until well combined, about 2 tablespoons.
Scrape the remaining garlic butter mixture into the cubed squash. Use a wooden spoon or rubber spatula to combine and coat all of the squash well. Turn out into the prepared baking dish. Arrange so the squash is evenly distributed from side to side. Season with the Malden salt and the black pepper.
Scrape the panko gremolata over the squash cubes. Use the tines of a fork to lift some of the cubes allowing the gremolata to fall between the crevasses of the squash.
Bake uncovered for 1 hour or until the squash cubes are sufficiently tender and the panko gremolata has turned a beautiful ochre color.
Serve immediately with additional fresh rosemary if desired.