1. Put a large, covered pan of salted water on to boil. While the water boils, peel and finely slice the onion. Bend the asparagus spears till they snap, then discard the woody ends (you can save these for your stock pot) and chop the tender spears into 2cm-long chunks.
2. Put a deep frying pan on a medium-low heat and warm for 2 mins, then add ½ tbsp oil and add the onion. Season. Gently fry, stirring often, for 8 mins till the onion is very soft and pale golden. If it starts to pick up too much colour too quickly, turn the heat down. Take off the heat and set aside.
3. While the onion fries, check the water’s boiling in the pan. If it’s boiling, add the spaghetti and simmer for 5 mins.
4. While the spaghetti simmers, finely grate the zest from the lemon. Pick the thyme leaves off the woody sprigs – you can keep any soft, white sprigs whole and just roughly chop them or tear them into small pieces.
5. Pop the thyme and lemon zest in a mixing bowl. Scoop in the mascarpone. Grate in ¼ of the nutmeg. Season with a little salt and pepper and stir to mix.
6. When the spaghetti has cooked for 5 mins, add the asparagus to the pan and simmer for 3-4 mins (don’t put the lid on the pan – this keeps the asparagus green). The spaghetti should be tender but still have a little bite. Scoop a cupful of water out of the pan and set aside, then drain the spaghetti and asparagus.
7. Stir 50ml of the pasta water into the mascarpone (about 3 tbsp). Stir till smooth and combined.
8. Put the onion pan back on the heat and add the spaghetti and asparagus. Toss to mix. Add the mascarpone sauce and stir to mix. Warm over a medium-low heat for 2 mins till warmed through. If it seems a little dry, add a splash more of the pasta cooking water.
9. While the spaghetti and sauce warm, whisk the balsamic vinegar with ½ tbsp olive oil and a pinch of salt and pepper. Toss the salad leaves with the dressing. Divide the spaghetti and sauce between 2 warm plates and serve with the salad.