Pat the 2 pounds flank steak strips dry and season with the 2 teaspoons salt and ½ teaspoon black pepper.
Heat the 3 tablespoons vegetable oil in a large heavy pot or Dutch oven over medium high heat. Sear the beef strips in batches for 2 to 3 minutes, until browned on the outside. Do not crowd the pot, since that steams the meat instead of searing it. Transfer the beef to a plate.
Lower the heat to medium and add the diced onion to the same pot. Cook for 3 to 4 minutes, until softened, scraping up the browned bits. Stir in the 5 cloves minced garlic and 1 tablespoon grated ginger and cook for 1 minute more.
Stir in the 3 tablespoons red curry paste and 1 tablespoon curry powder and cook for 1 minute, until fragrant and the paste darkens slightly. Blooming the paste in the oil is what wakes up all that flavor.
Pour in the coconut milk and 2 cups beef broth, then whisk in the ⅓ cup peanut butter, 2 tablespoons soy sauce, 1 tablespoon fish sauce, and 1 tablespoon brown sugar until smooth.
Return the beef and any juices to the pot. Bring to a gentle simmer, then reduce the heat to low, cover, and simmer for 1 hour 15 minutes to 1 hour 30 minutes, stirring occasionally, until the beef is fork tender. If the sauce gets too thick, loosen it with a splash of broth.
Stir in the 2 tablespoons lime juice. Taste and adjust the salt, lime, or sugar as needed.
Serve over the 3 cups warm jasmine rice, topped with the ⅓ cup chopped peanuts, sliced green onions, and ¼ cup cilantro.