In a wide pot, heat 1 tablespoon cooking oil over medium heat until very hot. Add the diced onion and cook, stirring regularly, for 5 minutes until softened and beginning to turn golden brown.
Add the garlic cloves and the tomatillos and cook for an additional 2-3 minutes.
Pour in the water along with the chipotle peppers and adobo sauce. Season with salt and pepper and bring to a boil. Reduce heat and simmer for 15 minutes or until the tomatillos are very soft and the liquid has reduced slightly. Turn the heat off.
Transfer the mixture to a blender, food processor, or use an immersion blender and blend until completely puréed. Transfer to a large bowl. Carefully wipe out the pot.