Adjust an oven rack to the top position and preheat the broiler. Line a rimmed baking sheet with foil.
Remove husks from the tomatillos and rinse well to remove the sticky residue; pat dry.
Arrange the tomatillos, serrano chiles, and garlic on the prepared sheet. Broil until blistered and blackened in spots, 5–7 minutes; flip and broil the other side until softened and charred in places, 4–6 minutes. (Alternatively, char everything in a dry skillet or comal over medium-high heat, turning until evenly blistered, 8–10 minutes.)
Transfer the roasted tomatillos, serrano chiles, and garlic to a blender; peel the garlic. Add the white onion, cilantro, and salt. Blend to a mostly smooth but slightly textured salsa, 20–30 seconds, scraping down as needed.
Heat the vegetable oil in a medium saucepan over medium-high heat until shimmering, 1–2 minutes.
Carefully pour in the blended salsa; it will sizzle. Bring to a simmer and cook, stirring, until slightly thickened and the raw edge mellows, 3–5 minutes, until it lightly coats a spoon.
Let cool 10 minutes, then taste and adjust seasoning if needed. Serve warm or at room temperature.
Store refrigerated in a sealed container for up to 5 days; stir before serving.