Cut off the top ends of the beans, leaving the cute curly ends, and steam them over a pan of salted boiling water for 6 minutes.
Whilst the beans are cooking, combine the mustard, vinegar, oil, lemon zest and juice, and the garlic in a jug or a jar.
As soon as the beans are cooked, drain and tip them into your serving dish. Immediately pour over the dressing. Give everything a good mix, and then sprinkle on the grated parmesan.
Season with a little sea salt (the parmesan is already quite salty) and some freshly ground black pepper and serve whilst still warm.
Notes
This dish isn't really one you can make ahead of time. It's definitely best served immediately while the beans are still warm and crisp. However, you can mix the dressing in a jar ahead of time so it’s ready to pour the second the beans are cooked!
Frozen beans don't really work here; ideally you need to seek out fresh beans.
If you don't have parmesan, you can use pecorino. Or, if you're looking to keep this vegan, then a suitable cheese alternative, or a sprinkling of toasted pine nuts and nutritional yeast gives a lovely savoury crunch!