Heat the oil in a large, deep frying pan over a medium heat. Add the onions and cook for 8-10 minutes, stirring often, until they soften. They should still hold their integrity in the final dish, so don’t cook them until collapsing.
Add the sliced asparagus to the pan and toss well to combine. Keep stirring for 1 minute, then add the stock. Simmer for 3 minutes.
Once the asparagus is tender, remove from the heat and add the lettuce, turning to coat with the cooking liquid and wilt the leaves. Season to taste. Finally, stir in the mint and serve.
