2) Pour the toasted seeds straight into a coffee grinder and blend the spices to a fine powder. Press all the spices, garlic and salt well into the lamb. Form into six even burgers.
3) For the sauce, rip the dried peppers open to shake out the seeds (a few seeds left in is ok). Throw the peppers in a saucepan with the water, and boil over a medium heat until only 60ml of liquid remains at the bottom.
4) Put the peppers (with the cooking liquid) into a blender and add the sour cream, oil, garlic and lime; blend it until very smooth. Add the salt and blend again. Add the cream and pulse a few times just to incorporate and get a saucy, squirtable consistency.
5) Grill the lamb burgers and the ciabatta buns until nice, dark grill marks appear everywhere (lamb is at its juiciest around medium rare). Put about one tsp of sauce on each side of the bun. Slice the burgers in half to lay lengthwise on the bread, and crumble Cotija cheese on top.
6) Dress with plenty of juicy tomato slices, onion and lettuce. Cut the sandwich lengthwise (along the seam of the lamb burger) and plate with cross-section facing outward.