Stir soy sauce, maple syrup, cider vinegar and liquid smoke together in a small bowl. Add sliced mushrooms and toss to coat. Allow to marinate for 30 minutes, tossing occasionally to recoat.
While the mushrooms marinate, bring a large pot of water to a boil and cook spaghetti according to package directions, until al dente. Reserve 1/2 cup of pasta water, then drain pasta into a colander. Return to pot and toss with a bit of olive oil.
Coat the bottom of a large skillet with 2 tablespoons of olive oil and place over medium heat. When oil is hot, add mushrooms in as even a layer as possible. Allow to cook until browned on bottoms, about 5 minutes. Flip and allow to cook 5 minutes more on opposite sides. Remove mushrooms from skillet and transfer to a plate.
Add 2 tablespoons more of oil to skillet. When oil is hot, add garlic and cook 2 minutes, until very fragrant. Add spaghetti, cashew mixture, peas, parsley and mushrooms to skillet. Toss everything a few times, until noodles are coated and ingredients are well mixed, adding some reserved pasta water to thin the sauce if needed. Season with salt and pepper to taste.
Divide onto plates and sprinkle with red pepper flakes and Parmesan if desired. Serve.