Roughly tear up the sourdough and put in the small bowl of a food processor. Whizz to breadcrumbs and set aside. Slice each asparagus spear on a steep diagonal into pieces about 3-4cm long; set aside.
Heat 2 tbsp oil in a frying pan. Pat the capers dry; add to the pan. Cook over a medium heat for 2-4 minutes, turning occasionally, until starting to crisp. Scoop out with a slotted spoon; drain on kitchen paper, leaving the oil in the pan. Add the breadcrumbs to the pan and stir over the heat until golden brown (4-6 minutes); set aside to cool and crisp up.
Meanwhile, cook the pasta following pack instructions, adding the asparagus pieces for the final 3 minutes. Put the garlic in a pestle and mortar with the lemon zest, chilli and anchovies. Pound into a smooth paste, then stir in the lemon juice and remaining oil. Drain the pasta and return to the pan. Toss with the anchovy paste, capers and parsley; season if liked. Divide between bowls, scattering with the breadcrumbs to serve.
