Preheat oven to 300F.

Place the hazelnuts on a rimmed baking sheet and toast the hazelnuts about 15 minutes, stirring occasionally, until fragrant and golden. Remove from oven and set aside.

When cool enough to handle, place 1 cup of the toasted hazelnuts in the bowl of a food processor or in the pitcher of a high-powered blender. Add the garlic, tahini, lemon juice, 1/4 cup of the olive oil, and a little of the warm water. Process until a puree forms, adding up to a total of 1/4 cup of water to achieve a creamy sauce. Season with salt and pepper to taste. Transfer to a small saucepan over low heat, and keep warm.

Coarsely chop the remaining 1/4 cup of toasted hazelnuts, set aside.

Cook the spaghetti in a large pot of boiling water, according to package directions. Drain, reserving some of the cooking water.

Heat the remaining teaspoon of olive oil in a large skillet over medium-high heat. Add the Brussels sprouts, in batches if necessary, and a pinch of salt and pepper. Cook undisturbed for 1 minute, until they begin to brown. Use a spatula to toss, and continue to cook for 2-3 minutes, until golden brown.

Combine the spaghetti and Brussels sprouts in a large bowl. Drizzle with some or all of the sauce, and toss to coat, adding a little pasta water if needed. Serve topped with the chopped hazelnuts and a pinch of crushed red pepper flakes.