Brown onions. Heat oil in a large frying pan over medium-high heat. Add onions and season with salt. Cook, stirring frequently, until tender and golden, about 5 minutes.

Brown remaining vegetables. Add garlic, arbol chiles, and tomatillos. Season again with salt and cook until chiles have darkened in color and garlic is browned, about 7 minutes more.

Blend. Transfer vegetables to a blender. Add water, measured amount of salt, and cilantro and blend on high until very smooth.

Taste. Dip a chip into the salsa. It should run off and leave a slick of salsa on the chip. If it is gloppy, add another tablespoon or so of water. Taste the salsa. It should pop with flavor. If it doesn't add a pinch or two more of salt.