When the liquids are boiling add the silver beet.
To roast lamb: Preheat oven to 200C fan forced. Coat lamb back strap in olive oil, sprinkle with salt and pepper. Place on a lined baking pan and roast on centre shelf for 25 mins or until medium rare. Set aside to cool. When completely cooled, slice half the back strap thinly across the grain and gently coat slices with hoisin sauce in a bowl; marinate for 15 mins. Reserve the remaining lamb: add to a salad, as a filling in wraps or sandwiches.
To prepare soup: In a large saucepan, heat the oil over a low – medium heat. Add the garlic, ginger, chilli and white spring onions and saute gently without browning until fragrant and tender. Slowly pour in one and a half cups of water, chicken stock, soy sauce and fish sauce. Bring to a simmer. Add the noodles, bring to a boil and add the greens. Cook for 3-4 minutes until noodles are soft.
Divide the hoisin lamb between two deep bowls. Using tongs, add the noodles to one side of the lamb, add the greens to the other side. Gently ladle the soup over the top, making sure to drench the lamb well in hot liquid to warm it through.