Preheat your oven to 400°F (200°C). Lightly grease a medium casserole dish (about 2 quarts) and set it aside.

Place the cubed butternut squash on a baking sheet. Drizzle with the olive oil, then sprinkle with about 1/2 teaspoon of the sea salt and 1/4 teaspoon of the black pepper. Toss well to coat the squash evenly and spread it into a single layer.

Roast the butternut squash in the preheated oven for 25 to 30 minutes, stirring once halfway through, until the cubes are very tender and starting to brown at the edges. Remove the tray from the oven and let the squash cool for a few minutes.

Transfer the roasted squash to a large mixing bowl. Using a potato masher or a fork, mash the squash until mostly smooth, leaving a few small chunks if you like a bit of texture.

Add the whole milk, sour cream, beaten egg, garlic powder, dried thyme, ground nutmeg, the remaining 1/2 teaspoon sea salt, and the remaining 1/4 teaspoon black pepper to the mashed squash. Stir until everything is well combined and the mixture looks creamy and uniform.

Fold in 1/2 cup of the shredded sharp cheddar cheese, reserving the remaining 1/4 cup for the topping. Mix gently until the cheese is evenly distributed through the squash mixture.

Spoon the butternut squash mixture into the prepared casserole dish. Spread it out with a spatula to create an even layer, smoothing the top slightly.

In a separate small bowl, combine the panko breadcrumbs, grated Parmesan cheese, melted butter, and the reserved 1/4 cup shredded cheddar cheese. Stir with a fork until the breadcrumbs are evenly moistened and the mixture is crumbly.

Sprinkle the breadcrumb and cheese mixture evenly over the top of the squash in the casserole dish, covering the surface from edge to edge.

Return the casserole dish to the 400°F (200°C) oven and bake for 15 to 20 minutes, or until the topping is golden brown and crisp and the squash mixture is bubbling gently around the edges.

Remove the casserole from the oven and let it rest for about 10 minutes to set slightly. If using, sprinkle the chopped fresh parsley over the top just before serving. Serve warm, scooping portions straight from the dish.