Halve the green beans and cut the asparagus (if using) into 3 pieces on the diagonal. Cook the tagliatelle following pack instructions, then add the veg for the last 5 minutes.
Grate the zest from half the lemon and squeeze the juice from the whole lemon. Put juice in a small pan with the mustard, oil and black pepper. Warm through until smooth.
Drain the pasta and veg, keeping 4tbsp of the cooking water (stir it into the mustardy sauce), then return the pasta to the pan. Reheat the sauce, adding most of the chives, and add to the pasta, tossing everything together well.
Divide between 4 shallow warmed bowls and top each one with a grind of black pepper, some Parmesan and the remaining chives.
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