Preheat oven to 400°F (204°C) and line a large baking sheet with parchment paper. Toss the sliced butternut squash with olive oil, sea salt, black pepper, and a pinch of cinnamon. Spread in a single layer and roast for 20 to 22 minutes, flipping halfway through, until tender and caramelized on the edges. Set aside.

While the squash roasts, melt 1.5 tablespoons butter in a skillet over medium-low heat. Add the sliced onions and a pinch of salt. Cook slowly for 15 to 18 minutes, stirring frequently, until deeply golden, sweet, and jammy. Stir in balsamic glaze for the final minute, then transfer to a small bowl.

Lay out the sourdough bread slices. Spread 1.5 tablespoons of creamy goat cheese generously across the bottom slices. Layer on the warm roasted squash slices, caramelized onions, shredded fontina, baby arugula, fresh sage leaves, and a light drizzle of hot honey. Top with the remaining bread slices and spread softened butter evenly across both outside faces of the sandwiches.

Heat a heavy cast iron grill pan or panini press over medium heat. Place the sandwiches into the hot pan and press firmly down with a cast iron press or heavy skillet. Grill for 3 to 4 minutes per side until the sourdough is deep golden-brown with crispy grill ridges and the cheeses are molten and bubbling. Slice diagonally and serve warm with extra hot honey.