Pat the shrimp dry. Blot with paper towels so they sear well.
Season with a big pinch of salt and black pepper.
Warm the pan. Heat a large skillet over medium-high. Add 1 tablespoon butter and the olive oil.
Sear the shrimp. Lay shrimp in a single layer. Cook 1–2 minutes per side until just opaque and lightly golden.
Transfer to a plate; do not overcook.
Build the aromatics. Lower heat to medium. Add the remaining 1 tablespoon butter. Stir in garlic and red pepper flakes.
Cook 30–45 seconds until fragrant, not browned.
Deglaze. Pour in the stock and scrape up any browned bits with a wooden spoon. Simmer 1–2 minutes to reduce slightly.
Make it creamy. Reduce heat to medium-low. Stir in the cream and bring to a gentle simmer.
Let it bubble softly for 2–3 minutes until it starts to thicken.
Add Parmesan. Sprinkle in the cheese gradually, stirring until smooth. The sauce should be glossy and coat a spoon.
Brighten. Stir in lemon zest and 1 teaspoon lemon juice. Taste and adjust salt, pepper, and lemon to your liking.
Optional veggies. If using spinach or tomatoes, fold them in now.
Cook 1–2 minutes until just wilted or warmed through.
Return the shrimp. Add shrimp and any juices back to the pan. Toss to coat and warm through for 30–60 seconds. Do not boil.
Finish and serve. Sprinkle with parsley.
Serve immediately over pasta or rice, or with bread. Top with extra Parmesan and a crack of black pepper.