Finely slice the red onion, garlic and mushrooms.
Heat a Tbsp of olive oil in a large frying pan. Add the onion and garlic, then gently sauté over medium heat until the onion is soft.
Add the mushrooms and cook for a further 5 to 8 minutes, under cover, until the mushrooms have wilted and released some liquid.
Remove and discard the woody ends of the asparagus. Cut the asparagus stalks into 3-4 (bite-size) pieces, then add these to the mushrooms. Stir in the lemon zest and juice; put the lid on, then gently cook for 2-3 minutes until the asparagus is tender but still has a bite.
Stir in the parsley, season to taste, then serve immediately with a dash of lemon juice.