The day before, put the chili peppers, peppercorns and coriander seeds along with 1 branch of fennel into the olive oil. Let sit overnight.
Select the best mussels and put them in a big pot.
Over the fire, let the mussels open, stirring them regularly. Once open, keep an eye on them. When the mussels are cooked, drain the excess water.
Remove the pot from the heat and pour the olive oil mixture over the mussels. Mix together well so that the olive oil coats the flesh of the mussels and cook another 2 or 3 minutes.
Serve immediately.