Method

Preheat the broiler to high. Roast whole tomatoes on a baking sheet 4 inches below a very hot broiler until the skins are blackened on one side, about 6 minutes, then flip and broil the other side. (Or roast tomatoes over a gas flame on the stove.) Allow to cool. Peel over a bowl to collect all the juices; discard peels. Pull out core of tomato, discard. Place tomatoes and their juices in a food processor or blender, and process to a coarse puree. Set aside.

Roast chiles directly over a gas flame or 4 inches below a very hot broiler, turning occasionally until blackened on all sides, about 5 minutes for open flame, about 10 minutes for broiler. Cover with a kitchen towel and let stand 5 minutes. Peel, pull out the stem and seed pod, then rinse very briefly to remove bits of skin and seeds. Slice into 1/4-inch wide strips.

In a medium-size (4-quart) pot heat the oil over medium to medium-high, then add the onion and cook, stirring regularly, until nicely browned but still a little crunchy, about 5 minutes. Add garlic and oregano, toss a minute longer, then stir in roasted chiles and heat through. Add tomato puree to the rajas and cook over medium-high, stirring frequently, until very thick and reduced, about 7 minutes. Stir in broth, partially cover and simmer over medium-low for 30 minutes.

Season to taste with salt. Serve the soup in warm bowls, topped with cubes of queso fresco.

Advance Preparation: The soup can be made a day or two in advance, but don't simmer it for the 30 minutes. Then, when reheating, cook the soup enough to blend the flavors, season with salt and serve with the cheese.