Use a vegetable peeler to peel your whole squash. Cut a thin slice off the top and bottom of the squash and stand it upright with the flat bottom surface down on a cutting board to stabilize the squash. Cut the squash in half with a very sharp knife, and scoop out the seeds, discarding the stringy bits.
Place the squash halves cut side down on a sheet pan lined with parchment paper. Brush the squash with oil and season it with salt and black pepper. Roast for 15 minutes.
Add the butternut squash seeds to a small baking dish. Toss with olive oil, salt, and pepper, and spread them into an even layer. Roast the seeds for 10-12 minutes (check after 10) then set aside to cool.
Remove the squash halves from the oven and allow them to cool for a few minutes before transferring them to a cutting board. Place wooden spoons or a pair of chopsticks on each side of the squash and make thin slices from end to end. Cut down until you hit the wooden utensil and then stop. This will help ensure you don't cut all the way through.
Carefully transfer the hasselback butternut squash back to the baking sheet and put it back in the oven for another 15 minutes.
Make the glaze by whisking together the olive oil or butter, maple syrup, vinegar, dijon mustard, minced sage leaves, and cayenne pepper in a small bowl. Season to taste with kosher salt and black pepper.
Use a pastry brush or a spoon to distribute half of the glaze over the squash and in between the hasselback slices. Cook for an additional 10-15 minutes.
Carefully transfer to a serving platter. Top with the rest of the maple glaze and sprinkle over the toasted seeds. Best when served warm out of the oven.