1/3 cup coarsely chopped cilantro stems and roots
2 cups water
3 to 4 tablespoons coarsely chopped garlic
4 ounces Fresno chiles, mostly seeded and coarsely chopped
1/8 teaspoon salt
About 2 cups distilled white vinegar
1 1/3 cups sugar
Put the cilantro stems and water into a sauce pan. Bring to a simmer and cook for 5 minutes. Remove from the heat and cover. Let steep for 20 minutes.
Meanwhile, use an electric mini chopper to grind the garlic, chiles, and salt to a coarse texture. Set aside.
Strain the cilantro liquid through a mesh strainer. Measure the liquid. You should have about 1 3/4 cups. Transfer to a saucepan. Add the same quantity of vinegar as you had of the cilantro liquid. Add the sugar and chiles and garlic mixture. Stir.
Bring to a boil over medium-high heat, then lower the heat to simmer. Let simmer until the volume has reduced by half. (How much time this takes depends on the size of your saucepan. Use a shallow, wide pan to hasten the process.) The resulting sauce should be slightly thick.
Remove from the heat and set aside, uncovered, to cool completely. Expect the sauce to thicken further and concentrate in flavor.