- Make the guajillo sauce. Set a heavy ungreased skillet or griddle over medium heat. Lay the unpeeled garlic on the hot surface and let it roast, until soft and blackened in a few spots, about 15 minutes. Cool, then slip off the papery skins and roughly chop.
While the garlic is roasting, break off the stems of the chiles, open them up, remove the seeds, then tear them into flat pieces. Toast the chiles a few at a time on a medium-hot skillet or griddle. Lay them skin-side up, press flat with a metal spatula until they are aromatic and lightened in color underneath – about 10 seconds per side. (If you see more than a whiff of smoke, they are burning.) Transfer the toasted chiles to a bowl, cover with hot water and let rehydrate for 30 minutes, stirring regularly to ensure even soaking. Pour off all the water and discard.
Transfer the drained chiles into a food processor or blender along with the garlic, oregano, black pepper and cumin. Measure in 2/3 cup of the broth and process to a smooth puree, scraping down the sides as necessary. If you’re using a blender and the mixture won’t move through the blades, add more broth a little at a time until everything is moving. With a rubber spatula, work the puree through a medium-mesh strainer into a bowl; discard the skins and seeds that remain in the strainer.
Heat the oil in a medium-size (4-quart) dutch oven over medium-high. When hot enough to make a drop of the puree sizzle sharply, add it all at once. Cook, stirring constantly, as the puree sears, reduces and darkens to an attractively earthy brick-red paste (usually about 7 minutes). You’ll know it’s cooked enough when it has lost that harsh raw-chile taste.
Stir in the remaining 3 cups of the broth, partially cover and simmer, stirring occasionally, about 30 minutes. If the sauce has thickened past the consistency of a light cream soup, add more broth. Taste and season with about 1 teaspoon salt and 1 1/2 teaspoons sugar--salt to brighten and focus the flavors, sugar to smooth any rough or bitter chile edges. Cover and set aside.
- Make the mushroom and swiss chard stuffing. Heat the oil in a large (12-inch) non-stick skillet over medium-high heat. When hot, add the mushrooms and 1/2 teaspoon salt. Cook for 8 to 10 minutes until golden brown. Stir in the minced garlic and cook for an additional minute. Add the chard and stir until wilted and dry about 5 minutes more. Remove from the heat and stir in 1/2 cup guajillo sauce. Cool to room temperature.
- Roast the tenderloin. Preheat oven to 425 degrees. Butterfly the tenderloin. Sprinkle with salt. Spread the cooled mushroom mixture over the surface leaving about 1/2-inch border around all sides. Roll up, as tightly as possible, and using kitchen string, tie every 1 1/2-inches, then tie once lengthwise. Push any stuffing that has fallen out back in. Rub the entire surface with olive oil and sprinkle with salt. Place on a rack, set over a rimmed baking sheet and roast for 30 to 40 minutes or until the internal temperature reaches 125 degrees for medium-rare. Remove from the oven, loosely cover with foil and let rest for 10 minutes before serving. While the meat is resting, warm the guajillo sauce.
- Serve. Remove the kitchen strings and cut into 1-inch slices. Place the slices on individual plates and spoon the warm guajillo sauce over and around the beef.