1. To prepare the pastrami spices; put all the ingredients except the garlic and oil into a blender and whizz to form a rough powder. Alternatively, use a pestle and mortar.
2. Transfer the powder to a bowl and add the garlic and oil. Rub the spice mix over the steaks. Cover and refrigerate for up to 2 hours, or if time allows overnight.
3. Remove the steaks from the fridge at least 1 hour before cooking.
4. To prepare the dressing; in a small bowl whisk the buttermilk into the mayonnaise, then stir through the shallots and herbs. Season. Cover and chill until required.
5. Heat a large non-stick frying pan and cook the steaks for 1-2 minutes on each side. Transfer to a large plate, cover loosely with foil and leave to rest for 5-10 minutes thinly slice.
6. Put the lettuce in a large bowl, add the steak and the dressing. Toss gently and serve with crusty bread.