I've playing around with this recipe for a while. For a number of reasons, firstly there is a pretty wide variety of approaches, some recipes use pork, others chicken, some are very simple and others have a large list of ingredients. Secondly the basis of the soup, the light broth enriched with a purée of tomato, onion and chile is common to lots of Mexican soups and a lot of Mexian recipes in general, and is something which I have been having difficulty with.
Seeing as so many sources which I regard as authoritative have different approaches to the dish, I picked the elements I have like best from what I have read, seen online and actually eaten in restaurants here. The dish I prefer is a soup of fried tortilla strips, served in a light broth enriched with tomato and garnished with cubed avocado, panela cheese, corriander and strips of passila chile.
The tomato base
We have seen this a number of time already, and the preparation is a fundamental to Mexican cooking as sofrito and mirepoix to Spanish and French cooking.
I have tried using raw tomatoes, onion and chile blended with water, which works fairly well and which I have seen used by lots of Mexican cooks online. The problem I find with this is that when the mixture is cooked out it tends to from a scum and it has a tendency to separate. I have also tried halving the tomatoes and cooking them on a dry non stick pan before blending them. This works a little better but still won't produce the light clear broth I am looking for.
The best approach would seem to be char the tomatoes under a hot grill (or broiler for any Americans), however while a have a lovely, and enormous, gas oven and a six burner hob in the apartment, I don't have a grill, so that approach is out.
The best compromise I have found, for soup at least, is to boil the tomatoes for five or ten minutes, skinning and coring them and then blending the flesh. When this is blended with the onion and chile the resulting mixture is homogeneous and cooks out to a beautiful golden colour and isn't inclined to foam.
Sopa De Tortilla
To make a single serving, I boiled a single large roma tomato, skinned and cored it and added it to the blender with a slice from a medium onion, a clove of garlic, about 3/4 of a serrano chile, salt and enough water just to allow the blades turn.
I cooked the mixture out in a small quantity of lard. This takes about 10 minutes and you need to be careful not to allow the mixture catch. While this was cooking I sliced two tortillas into strips 1/2" wide and fried them until golden in a small pan in an inch or so of corn oil. When they were golden brown I drained them on some paper and fried a garnish of passila chile strips in the oil.
I added chicken broth to the cooked tomato and brought the mixture to a gentle simmer.
I put the fried tortilla strips in the bottom of the bowl, topped them with a half of an avocado cut into cubes and the same quantity of queso panela. Then I poured the broth gently over all of this and garnished with the passila, some coriander and a drizzle of crema and served with a couple of wedges of lime.
It was damn nice.
The broth is refreshing, delicate yet spicy. It seemed odd at first to eat avocado in something hot but they add a lovely creaminess, and there is a hint of acidity from the crema.