Rinse chilies and tomatoes. Peel onion and cut into 3/4-inch-thick slices; rub salad oil over onion slices. Place chilies, whole tomatoes, and onion slices on a barbecue grill 4 inches above a solid bed of hot coals or over high heat on a gas grill (you can hold your hand at grill level for only 2 to 3 seconds); close lid on gas grill. (Or arrange vegetables in a 10- by 15-inch baking pan and broil 4 inches from heat.) Cook vegetables, turning as needed, until well browned on all sides, 8 to 12 minutes; as done, transfer to a board.
When cool enough to handle, peel, stem, and seed chilies; core and peel tomatoes. Chop chilies, tomatoes, and onions.
In a 10- to 12-inch frying pan over medium heat, stir chopped vegetables until hot, about 2 minutes. Add sour cream and stir just until hot but not boiling, about 2 minutes. Add salt and pepper to taste. Scrape into a bowl.