It's Time To Get Fat. Real Fat!
Spatchcock a 3-4 lbs roaster. In other words, cut out the spine using kitchen shears. Then splay the bird flat. This can be made easier by removing the wishbone.
Preheat oven to 425°F (400°F if using a convection oven).
Preheat a large (12 inch) cast iron frying fan.
Mix in a small bowl a spice mixture of crushed garlic, ground cumin, half the duck fat, smoked paprika, aji amarillo, aji rocoto, oregano, and the zest from 1 lemon.
Add to the spice mixture the juice from one large or two medium lemons. Zest the lemons before juicing.
Dry bird completely with cloth or paper towels. Arrange bird with breast up. Loosen the skin of the bird with your fingers being careful not to tear the skin.
Reach with fingers under the skin over the breast and thighs to create a space for the spice mixture. Spread the spice mixture under the skin of the bird. Wipe any stray mixture from the outside of the bird, and then rub into the skin salt and pepper mixed into the remaining duck fat.
Place the bird breast-skin side up into the preheated cast iron pan. Roast bird for 20 minutes on the middle rack of the oven, then rotate the pan 180 degrees.
Roast another 20 minutes and test the bird using a Thermopen or other instant-read thermometer. The thighs tested in multiple spots should register 165-170ºF. Return to oven if needed.
When done, transfer the bird to a cutting board or tray. Rest uncovered for 15 minutes. Collect any juices for au jus