- The soup. Lay the tomatillos and tomatoes on a baking sheet and set 4 inches below a very hot broiler. When blackened in spots and soft, about 5 to 6 minutes, flip and roast the other side. Cool; peel the tomatoes, collecting all the juice. On an ungreased griddle or heavy skillet set over medium heat, roast the garlic and chile: Turn the unpeeled garlic and the chile regularly until blackened in spots and soft, 10 to 15 minutes. While the garlic and chile are roasting, lay the onion out on a small piece of foil, set on the griddle or skillet, and dry-roast until deeply browned and soft, about 5 minutes per side. Let garlic cool, then slip off the papery skins.Combine all the roasted ingredients in a food processor or blender with the epazote, hoja santa (or anise seed), cilantro and 1 cup of the broth. Process to a smooth puree.
In a medium-size (4-quart) pot, heat 1 tablespoon of the oil over medium-high. When hot enough to make a drop of the puree sizzle sharply, add it all at once. Stir continually until darker and noticeably thicker, about 5 minutes. Add the remaining 3 cups of broth and the mushrooms, reduce the heat to medium-low and let gently simmer for 30 minutes, partially covered. Taste and season with salt, usually about a teaspoon (depending on the saltiness of the broth).
- The cactus and dried chile. While the soup is simmering, prepare the cactus: Holding a cactus paddle gingerly between the nodes of the prickly spines, trim off the edge that outlines the paddle, including the blunt end where the paddle was severed from the plant. Slice or scrape off the spiny nodes from both sides. Brush the paddles with the remaining 1/2 tablespoon oil and place them on a baking sheet. Set 4 inches below a very hot broiler or on a preheated gas grill and cook, turning the paddles occasionally, until limp, 10 to 15 minutes, depending on the heat of your fire. Cool, cut in half lengthwise, then cut each half crosswise into 1/4-inch slices.Turn the oven to 325 degrees. Spread the chile strips on a small baking sheet and toast in the oven until fragrant and lightly crisp, about 5 to 8 minutes.
- Serve the soup. Just before serving, add the cactus to the soup and bring to a boil. Ladle into warm soup bowls and sprinkle with the toasted chile.
- Advance Preparation: The soup can be made through Step 2 several days ahead and refrigerated, covered. Then reheat it and finish Step 3 to serve.
- Shortcuts: Purchased broth can replace homemade and the cactus could be replaced by an additional 1/4 pound of mushrooms.
Variations and Improvisations
Creamy Potato-Mushroom Soup with Bacon and Cilantro: Prepare the soup as described, omitting the herbs and adding 8 ounces diced Yukon Gold potatoes along with the mushrooms. Omit the cactus. Just before serving, stir in 2/3 cup chopped cilantro and 1/2 cup Mexican crema or heavy cream. Serve each bowl topped with a sprinkling of crumbled cooked bacon, chopped cilantro and pasilla chile.