Cook pasta in salted water until al dente. Reserve 1/2 cup pasta water before draining.
Cook chopped bacon in a large skillet over medium heat until crispy. Remove excess fat if needed, leaving 2–3 tbsp in the pan.
Add garlic and white scallion parts. Cook 30 seconds until fragrant.
Add chili flakes and black pepper. Stir briefly.
Add the al dente pasta to the skillet.
Add parmesan, then olive oil, then chili crisp or chili oil. Splash in pasta water gradually while stirring vigorously until the sauce is glossy and coats the pasta.
Toss in green scallions and serve immediately with extra parmesan.