Bring a large pot of salted water to a rolling boil, then add the penne pasta. Cook until just al dente, about 1 minute less than package directions. Drain and set aside, reserving a splash of pasta water.
While the pasta cooks, heat olive oil in a large skillet over medium-high heat. Once shimmering, add the shrimp in a single layer and cook for about 2 minutes, until they turn pink and opaque. Flip and cook another 1-2 minutes until fully cooked. Remove the shrimp from the skillet and set aside.
In the same skillet, add minced garlic and chili flakes. Sauté for about 30 seconds until fragrant, the garlic turns golden, and chili flakes crackle in the oil.
Return the cooked shrimp to the skillet. Squeeze in the juice of half a lemon and add lemon zest. Toss everything together, letting the flavors meld for about 1 minute. If the mixture looks dry, splash in a little reserved pasta water to loosen the sauce.
Add the drained penne pasta into the skillet with the shrimp and sauce. Toss gently over medium heat until everything is combined and heated through, about 1-2 minutes. Taste and adjust with more lemon juice or chili flakes if desired.
Turn off the heat and stir in chopped parsley for a fresh burst of flavor. Serve immediately, garnished with extra lemon wedges and parsley if you like.
