Place the dashi stock, half of the lemon zest and salt in a saucepan and bring almost to a boil, then reduce the heat immediately. Remove the zest with a sieve and discard.
Cut the asparagus spears in half and add to the saucepan. Bring to a boil and reduce the heat.
Add the scallops and soy sauce, stir, and simmer over low heat for another min