2 cups Avocado Oil or Grapeseed Oil
1 medium White Onion, quartered through stem end, layers separated
12–14 Garlic Cloves, smashed
8 Serrano Chiles, halved lengthwise, seeds and ribs removed
2 tbsp. Unsalted, Sugar-Free Creamy Peanut Butter
1 tsp. Vegetable Bouillon Concentrate, (preferably Better Than Bouillon)
½ tsp. Kosher Salt, plus more
Salted, dry-roasted Peanuts, chopped (for serving; optional)
Heat oil in a medium Dutch oven or other heavy pot over high. Add onion, garlic, and chiles. Reduce heat to maintain a low simmer and cook, stirring occasionally, until onions are translucent, chiles are blistered, and garlic is golden around the edges, about 10 minutes. Using a slotted spoon, transfer vegetables to a blender or food processor and let cool 10 minutes.
Add peanut butter, bouillon, and ½ tsp. salt to blender and purée. Remove small inset lid from top and, with the machine on the lowest setting, slowly pour in half the cooking oil (1 cup). Blend until salsa is velvety and emulsified. (Discard remaining oil or strain and reserve for another use, such as frying eggs or cooking the mushrooms for these tacos). Taste salsa and season with more salt if needed.
Transfer to a small bowl and top with peanuts just before serving.