Top and tail the beans, but leave them whole.
Soak them in very cold water for 5-6 minutes.
Drain the beans and steam them in a vegetable steamer for about 5 minutes, until just tender.
Alternatively, cook them in boiling water for 5 minutes.
The important thing is not to overcook them.
Meanwhile, spoon the mustard into a small bowl and add the salt, pepper, and lemon juice.
Stir to blend, then whisk in the oil.
Drain the beans.
Add the mustard sauce to them, toss to coat well, and serve immediately.