In a small bowl, combine the coarse salt and chopped thyme. Crush the bay leaves in a mortar and add them to the bowl with the sugar, coriander seeds, and pepper. Mince the garlic clove. Remove half of the fat from the skin of the duck breast. Take a piece of cling film measuring 12 x 16 inches (30 x 40 cm). In the centre, spread out half the seasoned salt and the garlic to cover an area the same size as the breast. Place the breast on it, skin downward. Cover with the rest of the salt and the garlic. Wrap the cling film tightly over the top. Transfer to a plate and leave in the lower part of the refrigerator for 4 days.
Remove from the refrigerator after 4 days, and take the duck out of the cling film. Rinse it well and pat dry. Wrap it tightly in burlap cloth or linen used for curing ham and truss it. Suspend it in the bottom of the refrigerator, or, better still, in a cool, well-ventilated cellar.
Eat 3 weeks later.