Trim the very top and bottom off the leek. Halve it and rinse out any grit. Finely slice it. Trim, peel and dice the potato. Dice the tomatoes. Tear the mushrooms into small pieces. Warm a large pan on a low heat for 1 min. Add 1 tbsp olive oil and the veg. Season with salt and pepper. Cover. Cook over a very low heat for 15 mins, stirring every few mins, till the veg are tender and glossy.
While the veg cook, fill and boil your kettle. Drain the borlotti beans and rinse them.
Tip the pasta and borlotti beans into the pan of veg and stir to mix. Crumble in the stock cube and drop in the bouquet garni (if you have kitchen string you can tie the herbs together to make them easier to fish out later). Pour in 1 ltr boiling water, pop a lid on the pan and bring to the boil. When the pan is boiling, turn the heat down a little and simmer for 5 mins till the pasta is half-cooked (it should still be a bit chalky if you bite into a piece).
While the soup simmers, shred the kale into ribbons, discarding any dry ends or thick pieces of core. Add the kale to the pan and stir to mix. Simmer, without the lid on, for 4-5 mins till the kale and pasta are tender.
Taste the soup and add a pinch more salt or pepper if you think it needs it. Ladle the soup into warm bowls, drizzle ½ tbsp vegan pesto over each bowl and serve.