I start by combining the shrimp with baking soda, kosher salt, black pepper, and crushed red pepper flakes in a bowl. I mix it gently and let it sit for about 10 minutes — this little brine helps keep the shrimp juicy and gives them great texture when cooked.
While the shrimp brines, I zest and juice the lemon, slice the garlic, and mince the parsley. Then I bring a large, salted pot of water to a boil and cook the spaghetti to one minute less than al dente. I make sure to reserve at least a cup of the pasta water before draining.
Next, I heat a large pan over medium heat with the extra-virgin olive oil. Once it's shimmering, I add the shrimp in a single layer and sauté for about 3–4 minutes, flipping halfway. It's important not to overcrowd the pan, so each shrimp cooks evenly and becomes firm and opaque but not rubbery.
When the shrimp is cooked, I remove them to a plate and tent them with foil to keep warm. Then, in the same pan, I add the sliced garlic, letting it sauté for 2–3 minutes until it's golden and fragrant. If the pan seems dry, I drizzle in a little more olive oil.
I pour in about ½ cup of the reserved pasta water, using a wooden spoon to scrape up the flavorful bits stuck to the bottom of the pan. Then I add the pasta and 4 tablespoons of butter, tossing for about a minute so everything emulsifies. If I can, I flip the pan to help the butter + pasta water cling to the noodles; if not, I stir very deliberately to coat well.
Finally, I add the shrimp back to the pan, along with the parsley, lemon juice, and lemon zest. I toss everything together, taste, and then adjust for salt, pepper, or more red pepper if needed. If the pasta seems a little dry, I loosen it with a splash more pasta water. Serve immediately for best texture and flavor.