Bring water to a boil in a small saucepan. Add walnuts and cook until they have softened slightly, about 5 minutes. Drain and set aside.

Combine garlic and pinch sea salt in the bowl of a mortar and pestle. Grind to create a thick paste. Add walnuts, marjoram, thyme, and oregano. Grind until combined and slightly creamy, but still coarse.

Transfer the walnut mixture to a large bowl. Slowly whisk in olive oil to form a thick emulsion. Add heavy cream and Pecorino Romano cheese and whisk until combined. Season with black pepper and sea salt to taste.

Fill a large pot with lightly salted water and bring to a rolling boil. Stir in fettuccine, return to a boil, and cook pasta over medium heat until cooked through but still firm to the bite, about 8 minutes. Drain.

Toss walnut sauce with pasta. Garnish with fresh chives.