Melt the butter in a wok or deep skillet over medium heat. Add the garlic paste and cook for about 1 minute until fragrant.
2 tablespoons unsalted butter, 4 ounces garlic paste
Add the oyster sauce, soy sauce, and fish sauce, and stir to combine. Remove from the heat.
4 teaspoons oyster sauce, 2 teaspoons light soy sauce or shoyu, 2 teaspoons fish sauce
Meanwhile, bring 1½ inches (about 4 to 5 cups) of water to a boil in a deep 12-inch skillet over high heat and season with salt. Add the pasta and stir so it doesn't clump. Reduce heat to medium and cook, stirring frequently, until just shy of al dente. The pasta will just fit lengthwise in the pan until it begins to soften and then you can stir it around. It's important to reduce the heat so the water doesn't evaporate too quickly and cause the pasta to burn.
1 pound spaghetti, salt
Using tongs, transfer the cooked pasta to the garlic sauce, along with whatever water clings to it. Reserve the pasta water in the skillet. Increase the heat to high, add the cheese to the pasta along with ¼ cup of the reserved pasta water, stir with a wooden spoon, and toss vigorously until the sauce is creamy and emulsified - about 30 seconds. If the sauce looks too watery, let it keep reducing. If it looks greasy, add a little more starchy water to it and let it re-emulsify.
⅓ cup finely grated Parmesan, 2 scallions
Stir in and top with the scallions and additional parmesan cheese, and serve immediately. Season with salt and pepper as desired.