Cook pasta in boiling salted water until it is al dente, drain off the water, and set aside.
In a large skillet over medium heat, add the garlic and onion. Sauté until fragrant, about 30 seconds.
(If needed, peel and devein shrimp) Season shrimp with old bay and toss until seasoning is spread evenly.
Add them to the skillet. Sear for about 1 min. Add butter. Cook until they are pink and cooked through, about 2-3 minutes per side. Remove the shrimp from the skillet and set aside.
Add the wine, seafood broth, lemon juice, parsley, and red pepper flakes (if using) to the skillet. Bring the mixture to a boil and let it simmer until it has reduced by about half, about 5-7 minutes. (If you want your sauce a bit thicker, add a cornstarch slurry 1tsp at a time. See slurry ratio in notes)
Add pasta to the skillet and toss it with the sauce until it's well coated. Drizzle with a little bit of olive oil and toss again.
Return the shrimp to the skillet and toss everything together until it's heated through.
Serve the shrimp scampi hot, garnished with freshly grated Parmesan cheese.