Preheat the oven to 400°F. Line a baking sheet with parchment or foil for easy cleanup.
Using a large, sharp knife, cut the squash in half lengthwise. Scoop out the seeds and inner membrane. Rub olive oil on cut sides of squash, then place cut-side down on the baking sheet.
2 to 3 pounds spaghetti squash, 1 tablespoon olive oil
Roast for 30-45 minutes, until tender (exact time will depend on size of your squash). Flip over to let cool.
While the squash is cooking, make the brown butter. Cut butter into pieces and add to a small saucepan over medium heat. Whisk as the butter melts and begins to foam, then continue whisking until brown specks start to form at the bottom and butter turns golden.
4 tablespoons unsalted butter
Remove butter from heat and pour into a bowl. Stir in the sage and garlic and set aside to let the flavors infuse.
5 whole sage leaves, 1 clove garlic
When the squash is easy to handle, use a fork to scrape into strands. Scoop into a bowl.
Discard sage leaves from the butter and pour butter and garlic over the squash in the bowl. Add chopped sage, hazelnuts, salt, and pepper, and use tongs to toss together.
1 to 2 teaspoons chopped sage, 1/4 cup hazelnuts, 1/2 teaspoon fine sea salt, Ground black pepper
Serve in a bowl or pile back into the squash skins, and garnish with extra sage and hazelnuts.