Prepare the grill for cooking over direct high heat.
Roast the poblanos until they are blistered on all sides, then set them aside in a covered container. Peel the onion and cut it into 3/4 inch slabs. Brush the slices with oil and grill until brown and soft, flipping occasionally. This should take 8 to 10 minutes total. Let cool.
Peel and chop the poblanos and put them in a medium bowl. Chop the onion and add it to the bowl. Add the tomatoes, pepper, cilantro, lime juice, and jalapeno. Mix well. Let rest for 15 minutes and mix again. Add salt as needed and mix again.