Thinly slice the zucchini. Thinly slice the spring onion, keeping the white and green parts separate. Crush or finely chop the garlic. Finely chop the dill, keeping the stems and leaves separate. Crumble 1 stock cube* into a heatproof jug, add 180ml (¾ cup) boiling water and stir to dissolve. Finely grate the zest of half the lemon*, then juice the half.
Step 2 — Prep salmon
Bring a medium saucepan of salted water to the boil for the pasta. To remove skin from the salmon, put the salmon on a board and, using a sharp knife, make a slight incision between flesh and skin. Hold the skin tightly (use paper towel for grip) and slice between flesh and skin in a downward motion, pushing the blade into the skin. Discard skin and cut salmon into 3cm chunks.
Step 3 — Make caper butter
Melt 30g butter in a small saucepan over medium-high heat. Cook the capers, stirring occasionally, for 2-3 mins until the butter is foaming and golden brown. Remove from the heat. Stir in 2 tsp lemon juice and 2 tsp extra virgin olive oil.
Step 4 — Start sauce
Melt 10g butter in a medium deep frypan over medium heat. Add the white part of spring onion, garlic, dill stems and 2 tsp flour and cook, stirring, for 1 min or until the spring onion is softened. Gradually add the stock and 125ml (½ cup) milk, stirring constantly to prevent lumps forming. Bring to a simmer and cook, stirring, for 2 mins or until slightly thickened.
Step 5 — Finish sauce
Add the salmon to the sauce, reduce the heat to medium-low and cook, covered, for 3-4 mins until the salmon is just cooked through. Remove from the heat. Stir in the lemon zest, 2 tsp lemon juice and 1 tsp dijon mustard. Taste, then season with salt and pepper. (The sauce may seem thin, but it will thicken when the pasta is added.)
Step 6 — Cook pasta and serve
Meanwhile, cut lasagne sheets into 1cm-wide strips (see cooking tip), then separate the strips. Add pasta, zucchini and peas to the pan of boiling water and cook, stirring occasionally, for 1½-2 mins until al dente. Drain, then toss through the sauce. Divide pasta among bowls. Scatter with dill and remaining spring onion. Drizzle with caper butter to serve.