Make the peanut sauce by combining 1/2 cup creamy peanut butter, 1/3 cup tamari or soy sauce, 2–3 tablespoons Thai red curry paste, 1 tablespoon chopped pickled ginger, and 1 tablespoon toasted sesame oil in a blender or jar; blend and add water as needed to reach a pourable consistency.
Toss the thinly sliced flank steak with a pinch of black pepper and 2 teaspoons cornstarch (or arrowroot) until evenly coated.
Heat 1 tablespoon of the butter in a large skillet over medium-high heat. Add the beef and 1/4 cup Thai red curry paste; cook, stirring occasionally, until the edges brown and become slightly crisp, about 5 minutes.
Add 2 tablespoons more butter, the chopped garlic, 1 tablespoon lemongrass paste (if using), and 2 tablespoons chopped pickled ginger; cook until the garlic is lightly crisping, about 1 minute.
Reduce heat to low and stir in 1/3 cup tamari or soy sauce and 2 tablespoons sweet Thai chili sauce; cook 2–3 minutes more until the beef is caramelized and sauce is slightly thickened.
Make the mango salsa by combining 1 cup chopped mango, the sliced seeded jalapeño, 1/2 cup chopped cilantro, 2 chopped green onions, 2 tablespoons sesame seeds, 2 tablespoons chopped peanuts, and 2 tablespoons lime juice in a bowl; stir to combine.
Serve by spooning mango salsa over rice, top with the curried beef, and drizzle the peanut sauce over everything.