Boil 2 cups of water. Place 5 small Guajillo peppers, cleaned of seeds and stems, in a bowl and cover with enough hot water. Let sit for 15-30 minutes until hydrated, then drain.
Cook onion in a cast iron pan until charred.
In a blender, blend together kumquats or mandarin oranges, hydrated peppers, charred onion, garlic, and seasonings. May add a touch of Guajillo pepper water to thin as blending.
Place paste into a bowl. Fold in the Greek yogurt. Add lemon zest.
Apply marinade to desired meat or vegetables.