Heat your oven to 200°C/Fan 180°C/Gas 6. Line a baking tray with baking paper. Slice each tomato into 4 slices. Lay them in a single layer on the baking tray. Drizzle over 2 tsp olive oil and scatter over a pinch of salt and pepper. Roast on the top shelf of the oven for 30 mins.
While the tomatoes roast, zest the lemon into a medium bowl. Squeeze in the juice from half the lemon. Peel and grate in the garlic. Whisk in 1 tsp mustard, 1 tbsp vinegar and 2 tsp oil. Finely chop the dill leaves and the parsley leaves and stalks. Add to the bowl and season. Mix. This is your salsa verde. If it's dry, add 1 tbsp water.
Place the salmon fillets on a plate. Squeeze over the remaining lemon juice and sprinkle over a little salt and pepper. Turn the fillets to coat. Set aside.
Using a vegetable peeler, peel long ribbons off the sides of the courgettes. Stop when you get to the seeded cores. You can add the cores to your stockpot or compost them.
Lay 6-8 ribbons of courgette side-by-side on a chopping board, overlapping each slice by half. Place a salmon fillet at one end and wrap it in the courgette ribbons. Place on a baking tray lined with baking paper. Repeat with the remaining courgette ribbons and salmon. If you have any ribbons left over, pop them on the baking tray next to the salmon.
When the tomatoes have cooked for 30 mins, slide the salmon onto a shelf below them. Bake the salmon for 10-15 mins till just cooked though. If your salmon fillets are flat and wide, they will only need 8-10 mins. Check by inserting a sharp knife into the thickest part of the salmon. It should be just opaque.
Put the watercress in a large bowl. Add any cooked courgette ribbons to the watercress. Pour over half the salsa verde and toss together.
Serve the salmon fillets with a tangle of watercress and the roast tomato slices, with the remaining salsa verde on the side for drizzling.