Stem and seed chiles, opening them flat. Heat a large cast-iron skillet over high heat.
Press 1 chile at a time onto hot pan, turning once and cooking until blistered, 3 to 5 seconds per side. Submerge in a bowl with 2 cups hot water, 45 minutes.
Whirl chiles and liquid in a blender with garlic cloves and salt until very smooth.
In a medium saucepan over medium-high heat, sauté onion in lard. Add chile sauce; cook at a low boil, stirring often, 5 minutes. Add 1 cup hot water, reduce heat, and simmer 10 minutes, or longer if you want it thicker.
*Buy chiles at well-stocked grocery stores and Latino markets.
Salsas and Moles (Ten Speed Press, March 31, 2015)