Warm a dry frying pan over a low heat. Add the breadcrumbs. Toast them for 1-2 mins. Keep shaking the pan till they’re golden. Keep your eye on them so they don't burn. Tip into a food processor or blender.
Remove any thick stalks from the watercress. Rinse and pat dry. Roughly chop. Add to the food processor. Put to one side. Fill the kettle and boil it.
Scrub and thickly slice the potatoes. Snap the woody stems off the asparagus stalks. Trim the asparagus into lengths around 3 cm long. Pop them to one side.
Heat your grill to high. Fill a pan with hot water from the kettle. Add a pinch of salt. Cover. Bring to the boil. Pop the tomatoes onto a baking tray or in a roasting tin. Drizzle with 1 tsp olive oil. Season.
Pop the tomatoes under the grill for 10-12 mins till they are soft and charred. When the water in the pan is boiling, add the spaghetti and sliced potatoes. Cover. Simmer for a very precise 7 mins.
Add the asparagus to the pan. Simmer for another 3-4 mins till the asparagus and potatoes are soft when you poke them with a fork or skewer.
While the spaghetti cooks, finely grate or pare the zest from the lemon. Juice it. Peel the garlic. Add to the watercress with 4 tbsp olive oil and a pinch of salt. Whiz to make a thick pesto – add more oil if you think it needs it.
If you don’t have a processor or blender, finely chop everything and mix together, bashing with the end of a rolling pin to make a chunky paste. Drain the pasta. Mix with a few spoonfuls of the pesto. Serve with the grilled tomatoes on top.