Mix yoghurt and milk together and leave on bench while you prepare salad.
Heat olive oil and butter in a large frying pan. Add bread and garlic and fry until golden and crispy (about 5 minutes). Season and toss with a good pinch of salt.
Place asparagus in a heatproof bowl or a pot and pour in boiling water to cover. Leave for a few minutes to lightly cook, then drain and run under cold water (or plunge into bowl of iced water) to refresh.
In a large bowl, toss asparagus, radish, cucumber, spring onion, parsley and mint with the lemon juice, a good drizzle of extra-virgin olive oil and the yoghurt dressing. Season to taste with salt and freshly ground black pepper. Gently toss through half the crunchy bread.
Serve immediately scattered with the remaining crunchy bread and sprinkled with sumac.