Remove husks and stems from Tomatillo (1 pound). Run them under warm water to remove the sticky residue. In a large pot, boil the tomatillos and Serrano Chili (1) in salted water for about 10-15 minutes until tender and cooked through.
Place tomatillos, chiles, Yellow Onion (1), Garlic (1 clove), and Fresh Cilantro (6 sprig) in a food processor. Blend until smooth, but retaining a little texture. Not liquified.
Heat the Vegetable Oil (1 teaspoon) in a large skillet over medium-high heat. When it’s hot enough to make a drop of the salsa mixture sizzle, add all of it to the pan at once and stir frequently for about 4 – 5 minutes until the sauce thickens and darkens.
Add the Chicken Stock (2 cup) and bring to a boil. Reduce the heat and simmer until the sauce is thick enough to coat the back of a spoon. Season with salt to taste.