Quick Pork Pho

Heat a dry large cast-iron skillet over medium-high. Cook onion, cut side down, until lightly charred, about 5 minutes; transfer to a plate. Add chiles (if using crushed red pepper flakes, add with fennel seeds), garlic, cinnamon stick, and star anise to skillet and cook, stirring, until fragrant, about 1 minute. Add fennel seeds and cook, stirring, until fragrant, about 20 seconds (do not burn). Quickly transfer to a large saucepan (reserve skillet) and add onion, ginger, and broth; bring to a boil. Reduce heat and simmer until broth is flavorful. Meanwhile, heat oil in reserved skillet over medium-high heat. Season pork with salt and pepper and cook until browned and cooked through, about 4 minutes per side. Transfer to a cutting board and let rest 5 minutes before thinly slicing. Cook noodles according to package directions. Divide among bowls; add pork. Strain broth and ladle into bowls. Top pho with bean sprouts, cilantro, scallions, and peanuts and serve with lime wedges.