For the Sauce and Filling Base
In a medium large (4 quart) saucepan, heat 3 tablespoons of the oil over medium heat. Add the onions and cook, stirring regularly, until starting to brown. About 10 minutes.
While the onions are cooking puree the undrained tomatoes in a food processor. Working in two batches if necessary.
When the onions are browned, raise the heat to medium high and add the tomato puree, cinnamon and pepper.
Stir regularly as the mixture boils briskly, reducing to the consistency of a thick tomato sauce. About 25 minutes. Remove from heat.
For the Filling
In a large 10 - 12 inch non-stick skillet heat the remaining 1 1/2 tablespoon oil over medium high heat. Add the mushrooms and cook stirring often until the edges turn golden. About 5 minutes.
Stir in half of the tomato mixture, the corn and vinegar and continue to cook over medium heat, stirring regularly until the mixture is thick enough to hold its shape on a spoon. About 15 minutes. Stir in the chicken, spinach and cilantro. Taste and season with salt, about 1 teaspoon.
The Tomato Sauce
Stir the broth into the remaining tomato sauce. Partially cover and simmer over low heat for 45 minutes. In the meantime prepare the chiles.
Preparing the chiles
Roast the chiles directly over a gas flame blistering and blackening on all sides. A pair of tongs work well to turn chiles. Be careful not to char the flesh, only the skin. Place the peppers in a gallon sized baggie, no need to seal, and let them steam for about 10 minutes.
Rub off the blackened skin, then cut an incision in the side of each one, starting 1/2 inch below the stem to 1/2 inch of the tip. One by one, work your index finger inside each chile and dislodge the seeds clustered below the stem. Quickly rinse the seeds from inside the chiles. Be careful not the rip the opening any wider. Drain cut side down on paper towels.
Stuffing and baking the chiles
Heat oven to 375 degrees. Fill each chile with about 1/2 cup of the chicken mixture and reform it into its original shape. Then lay seam side down in a 13 x 9 inch baking dish. Check the consistency of the tomato sauce. It should be medium thick. If it's too thick stir in a little water.
Pour the sauce over the chiles, covering them, then drizzle with the creama, creme fraiche or cream. Bake until the sauce is bubbling and the cream is beginning to brown. About 20 minutes.