Place onion and garlic in a food processor and process until a mulch is formed. Transfer to a bowl.
Add rosemary, thyme and sage and combine.
Using a mortar and pestle crush the peppercorns so that they are not a powder but resemble a fine kibble. Add this to the marinade with the olive oil and some salt.
Mix the marinade together forming a paste.
Massage marinade into the steaks and allow to sit covered in the fridge overnight (or at least 6 hours)