Bring a large pot of water to a rolling boil and salt it generously (about 1 tbsp salt per 4–5 liters/quarts). Salted pasta water seasons the pasta from the inside and improves flavor. Add the whole-wheat spaghetti and cook to al dente per package directions (usually about 7–9 minutes). Reserve 1 cup (240 ml) of the starchy cooking water before draining — that liquid is key for loosening and emulsifying the sauce.

While the pasta cooks, pat the shrimp very dry with paper towels and season lightly with salt and a pinch of black pepper. Drying the shrimp removes surface moisture so they sear quickly, giving a slightly caramelized edge without steaming.

Heat a large sauté pan over medium-high heat until hot, then add 1 tbsp of olive oil. When the oil shimmers, add the shrimp in a single layer and sear 1–1½ minutes per side (depending on size) until opaque with a faint golden edge. Do not overcrowd the pan — quick, high-heat searing develops flavor and prevents rubberiness. Transfer the shrimp to a plate and keep warm; they’ll finish cooking slightly when returned to the sauce.

Reduce the heat to medium and add the remaining 2 tbsp olive oil to the same pan. Add the sliced/minced garlic and red pepper flakes. Toast the garlic for about 30–45 seconds, stirring constantly — you’re blooming the garlic and red pepper to release aromatic oils. Watch carefully: garlic burns quickly and becomes bitter.

Add the halved cherry tomatoes to the pan and increase the heat to medium-high. Cook, stirring and pressing a few tomatoes with the back of a spoon so some juices escape and create a light sauce, about 3–4 minutes. This blistering/bursting concentrates sweetness and creates a natural pan sauce to cling to the pasta.

Add the lemon zest and most of the lemon juice (reserve 1 teaspoon for finishing) to deglaze the pan, scraping up any browned bits — those bits are flavor. Allow the lemon and tomato juices to come together for 30–60 seconds so the acid brightens the sauce without curdling the butter later.

Return the seared shrimp and any accumulated juices to the pan. Stir to combine and heat through for 30–60 seconds. Shrimp finish quickly; avoid long simmering to keep them tender.

Add the drained spaghetti directly to the pan with the sauce and toss vigorously. Add 2–4 tablespoons of the reserved pasta water as needed — the starch in the water helps the oil and lemon emulsify into a silky coating that clings to the pasta. Toss for 30–60 seconds until well coated and glossy.

Remove from heat and stir in the butter, chopped parsley, and the remaining teaspoon of lemon juice. The butter enriches and rounds the flavors and helps create a satiny finish; adding it off the heat prevents the sauce from breaking. Taste and adjust seasoning with salt and pepper.

Plate the pasta immediately, arranging shrimp on top. Finish with an extra drizzle of good olive oil, a pinch of red pepper flakes if desired, and a sprinkle of freshly grated Parmesan if using. Serve hot for best texture.