Using kitchen shears, snip stems from dried chiles and empty of any loose seeds. Don’t worry about removing every single seed, but removing some will reduce the chance of bitterness in your finished salsa. Snip chilies in half.
In a dry skillet over medium-high heat, toast chile halves for a few moments per side, just until fragrant. Do not burn.
In a small saucepan, combine the chiles, onion, garlic and 2 cups of water. Bring to a boil. Remove from heat, cover and let steep for 10 minutes.
Using tongs, fish chiles and onion out of saucepan and place them into the jar of a blender or immersion blender. Do the same for cloves of garlic, pausing to peel them out of their papery skins. Reserve 1/2 cup of cooking water.
Add half of the reserved cooking water to blender and puree. Add lime juice, oil, and salt and blend to combine, thinning with more cooking water as needed. Add more salt and lime juice if necessary.
Store salsa in the refrigerator and eat within a week.