This sauce is very versatile, and can actually be made using a variety of nuts.
Roughly chop the hazelnuts and garlic. Into a small pan, add the olive oil, chopped garlic and hazelnuts. Bring the stove to a medium heat, and gently stir and toss the garlic and hazelnuts in the oil until they begin to brown and become fragrant. This should take between 3 and 5 minutes. Do not let them burn, or they will turn bitter. When the garlic is about the same color as the flesh of the hazelnuts, remove the pan from the heat and allow them to stop sizzling in the pan.
You will be adding hot, salted pasta water to this later, so hold off on the salt for right now.