How to Make It
Heat a cast-iron skillet over medium-high, and add chiles. Press flat with the back of a spatula, and toast until slightly softened and a shade darker but not black, 10 to 15 seconds per side. Remove chiles from skillet, and let cool slightly. Wearing gloves if possible, trim stems; cut chiles open, and remove and discard seeds. Cut chiles into very small pieces. Place chile pieces in a small bowl, and add boiling water to cover. Let soak until soft and rehydrated, 10 to 15 minutes. Pour through a fine wire-mesh strainer into a bowl; discard liquid.
Using a molcajete, pound and grind cumin seeds and peppercorns to a fine powder, about 3 minutes. Add garlic; pound and grind to a thick paste, about 1 minute. Add half of the rehydrated chiles and 1 teaspoon salt, and pound until not much of the chiles are visible, about 4 minutes. Add 1 teaspoon salt and remaining rehydrated chiles, and pound until mixture is a thick, relatively smooth paste, about 4 minutes. Add orange juice and lime juice, a little at a time, stirring with pestle to combine. Taste and adjust seasonings if needed.
Season both sides of flank steak with remaining 2 teaspoons salt, and place in a shallow dish. Pour 1/2 cup chile sauce over steak, coating evenly. Set remaining chile sauce aside. Cover flank steak, and let stand at room temperature at least 2 hours or up to 4 hours. (Alternatively, marinate in refrigerator 8 hours. Let steak come to room temperature at least 1 hour before grilling.)
Preheat grill to high (450°F to 500°F). Without brushing off marinade, place steak on oiled grates, and grill, uncovered, until charred and a thermometer inserted into thickest portion registers 125°F, 4 to 6 minutes per side.
Remove steak from grill, cover with aluminum foil, and let rest 8 to 10 minutes (internal temperature will continue to rise). Cut steak against the grain into thin strips, and arrange on a serving platter. Serve steak with tortillas, remaining chile sauce, radishes, cilantro, and lime wedges.
Dried pasilla de Oaxaca chiles may be ordered online.