Mix the salt and sugar together. Using a plastic container that your fillet will fit in uncured, place five sprigs of rosemary and 5 sprigs of thyme in the bottom. Generously cover with the salt and sugar mix, lay the fillet on top, then cover with the rest of the mix. Nestle the rest of the rosemary and thyme into this. Cover the container and refrigerate for 3 days.
Remove the fillet from the cure, rinse under cold running water, and dry with a clean cloth. When dry, roll in the black pepper; this should remove any remaining moisture. Wrap in plastic and keep in the fridge until needed.