Preheat oven to 425°.
Place onion, jalapeños, poblano chile, and tomatillos on a large rimmed baking sheet; drizzle with olive oil and season with salt. Toss to coat and arrange in an even layer.
Roast until tomatillos burst and are jammy and onion and chiles are tender and blistered in spots, 17–22 minutes. Let cool.
Transfer vegetables to a food processor. Add avocados, garlic, cilantro, and lime juice and process until smooth. Taste salsa and season with more salt if needed.
Transfer salsa to a small bowl and serve with plantain chips for dipping.
