Whisk the soy sauce, hoisin, water, wine, sugar, sesame oil, red pepper flakes, and salt in a small bowl until the sugar begins to dissolve. In a separate small container, stir the cornstarch and room-temperature water together to create a smooth slurry.

Boil the pasta in a pot of water according to package instructions, but drain it while it is still very firm and chewy—it will continue to soften as it simmers in the sauce.

Heat the toasted sesame oil in a large skillet over medium-high heat until it shimmers. Sauté the garlic, onion, and scallion for about 1 minute until the aromatics are fragrant and the onions are just beginning to soften.

Stir in the mushrooms and cook until they are tender and have released their moisture, about 3 minutes.

Pour the prepared sauce mixture into the skillet and bring it to a boil over medium heat. Give the cornstarch slurry a quick stir to recombine, then whisk it into the sauce, stirring continuously until the liquid thickens and turns glossy.

Toss the drained pasta into the skillet and simmer over high heat for 2 to 3 minutes, stirring well to ensure the noodles are evenly coated and have absorbed the flavors of the sauce.

Fold in the roasted cashews and serve immediately while hot, topped with fresh scallions.