Instructions

• Make the lemon-garlic sauce: in a blender combine the juice and zest of both lemons, the minced garlic, and the toasted walnuts. Blend on high until smooth, about 30 seconds. Add the grated Parmesan and 3 tablespoons of the olive oil and blend again until well combined. Transfer the sauce to a small bowl and set aside.
• Bring a large pot of salted water to a boil. Add the spaghetti and cook according to package instructions until al dente. Before draining, scoop out and reserve 1 cup of the pasta cooking water. Drain the pasta and set it aside.
• Heat a skillet over medium-high heat and add about 1 tablespoon of the olive oil. Add the baby capers and cook, stirring occasionally, until they are crisp, about 2–3 minutes. Use a slotted spoon to transfer the capers to a plate lined with paper towels to drain.
• In the same skillet, add about 1 tablespoon more of the olive oil and reduce the heat to medium. Add the sourdough bread cubes and cook, stirring or tossing occasionally, until golden brown and crisp, about 2–3 minutes. Transfer the toasted bread cubes to the same plate as the capers.
• Pat the seasoned salmon pieces dry with paper towels. In a clean skillet, melt the unsalted butter over medium-high heat. Add the salmon pieces in a single layer (work in batches if needed) and cook until opaque and fork-tender, about 3–5 minutes total. Remove the salmon from the skillet and set aside.
• Return the skillet (wipe out first if very browned) to medium heat. Add the prepared lemon-garlic sauce, the drained pasta, and up to 1 cup of the reserved pasta water. Toss or use tongs to coat the spaghetti with the sauce, adding only as much pasta water as needed to reach a smooth, glossy consistency (usually all or most of the reserved cup), about 1–2 minutes.
• Gently fold the cooked salmon pieces and the watercress leaves into the sauced pasta, tossing carefully so the salmon stays in pieces and the watercress wilts slightly, about 30–60 seconds.
• Divide the pasta among plates or a serving bowl and top with the crispy capers, toasted sourdough cubes, and any remaining watercress. Serve immediately.