Combining fresh chorizo sausage (squeezed out of its casings) with plain ground pork makes for a spicy, brawny burger. You can serve these plain, but I live to caramelize sliced onions on the grill and pile them on top. Their sweet-and-sour notes go really nicely with the pork. And a few slices of Manchego cheese add a pleasing salty creaminess that keeps with the Spanish theme here, though any sheep's milk cheese will work. One thing to note: because the burgers are made from raw pork, don't serve them rare; the fat content of the sausage and pork will ensure that your burger will stay nice and juicy even when cooked to medium. And be sure not to use cured chorizo, the salami-like kind you can slice and eat without cooking. The leathery texture won't work in a burger unless you pass it through a meat grinder.
If you can't find fresh chorizo, you can substitute spicy Italian sausage. Just increase the smoked paprika by one teaspoon. You can form the burger patties the day before grilling; wrap them well and keep them in the fridge until you're ready to light the grill.