Peel and thinly slice the garlic. Finely grate the zest from the lemon and juice one half (keep the other half for later). Quarter the cherry tomatoes. Trim the roots off the spring onion and finely slice it. Pick the leaves from the basil.
Heat a large, heavy-based pan for 2 mins. Tip in the pine nuts and toast for 2-3 mins till golden. Tip out into a bowl. Fill the pan with water and bring to a boil.
While the water is coming to the boil, pop the broad beans from their pods by pulling the pods open and scooping out the beans. When the water is bubbling, drop in the beans. Cook for 2 mins. Drain, catching 500ml water in a jug. Plunge the beans into ice cold water to stop them cooking further.
Pour the reserved 500ml water back into the pan and bring back to the boil. Pour in 3 tbsp oil and add ½ tsp salt. Add the garlic, lemon zest, lemon juice and all the cherry tomatoes.
Snap the spaghetti in half, so it sits flat in the pan. Add it to the pan. Clamp on a lid till the water comes back to the boil again. Remove the lid. Simmer on a high heat for 8 mins. Toss the pasta every now and then while it cooks.
Meanwhile, pop the broad beans from their grey skins. The easiest way to do this is to pinch one end and squeeze so they pop out. This is optional: when broad beans are young like this the grey skins aren’t too tough.
After 8 mins, the spaghetti should be soft with a slight bite and most of the water evaporated. Throw in the broad beans and the spring onions. Toss together for 1 min to warm through.
Add most of the basil and fold it through the spaghetti with a crack of pepper. Add more salt if needed. Tangle into warm bowls, spooning over any sauce and the veg. Top with the remaining basil, the watercress and toasted pine nuts. Serve with lemon wedges.