Put a dry pan onto a medium heat and warm up for 2 mins. Scatter in the pine nuts and cook for 2-3 mins, tossing frequently, till toasted. Tip into a bowl.
Peel and grate 1 garlic clove into a food processor. Grate in the lemon zest and squeeze in half of the juice. Add most of the rocket leaves (save a small handful for later). Scatter in most of the pine nuts and crumble in most of the half-pack of feta. Add a crack of pepper and 1 tbsp oil. Whizz into a smooth pesto. Have a taste and add a little more pepper or salt if needed.
Peel the red onion and thinly slice it. Peel and crush or finely chop the remaining garlic clove. Scrub the potato and cut into bite-sized pieces.
Strip the kale leaves from the thick stems. Discard the stems and finely shred the leaves.
Fill a pan with boiling water. Sprinkle in a generous pinch of salt and return to the boil. Slide in the potatoes and boil for 5 mins, then add the spaghetti. Cook for a further 8 mins.
As soon as the pasta is cooking, pour 1 tbsp oil into a large pan and brig to a medium heat. Slide in the onion and garlic and fry gently for 3-4 mins, till slightly softened. Stir in the kale and cook for a further 3-4 mins, till wilted.
When the pasta and potatoes are cooked, drain through a sieve with a bowl underneath to catch the water. Add the pasta and potatoes to the kale and onions, along with a splash of the cooking water. Add the pesto and stir really well, till the sauce thickens and coats the pasta and potatoes. Have a taste and add more salt and pepper if you think the dish needs it.
Pile the spaghetti onto plates and crumble over the remaining feta. Scatter over the reserved pine nuts and rocket leaves. Squeeze over the rest of the lemon juice and serve.