Clean and rinse the mussels under cold water.
Melt butter in a large pan over medium heat. Add the diced onion and cook for a few minutes until translucent.
Add the nduja paste, breaking it up with a spoon as it melts into the pan.
Pour in the vermouth, deglaze the pan and reduce by half.
Add the mussels, cover with a lid and allow to steam for 2 minutes. Shake the pan occasionally to move the mussels through the sauce.
Add the cream, cover again and cook for another 2–3 minutes.
Stir through the chopped parsley.
Check the mussels are cooked (they should be open, plump and juicy).
Remove from heat, squeeze over fresh lemon and serve with bread.