Soak the rice noodles according to the package directions until tender, then drain and set aside.
Whisk together the tamari (or soy sauce), Thai chili sauce, fish sauce, rice vinegar, lemongrass paste (if using), a pinch of black pepper, and chili flakes in a bowl to make the sauce.
Heat 2 tablespoons sesame oil in a large skillet or wok over medium-high heat. Add the sliced flank steak and bell peppers and cook, stirring occasionally, until the beef develops some browning and the peppers soften, about 4–6 minutes.
Push the beef and peppers to the side, add 1 tablespoon butter to the skillet, then add the sliced shallot, 3 cloves chopped garlic, and the chopped pickled ginger. Cook, stirring, until the garlic is fragrant and just turning golden, about 1–2 minutes, then toss everything together.
Add the drained noodles and the prepared sauce to the skillet. Toss or gently stir to combine and cook until the noodles are heated through and begin to absorb the sauce, about 1 minute. Remove from heat.
Stir in the chopped cilantro or Thai basil and the chopped green onions, tossing to distribute evenly.
In a small skillet, heat 1 tablespoon sesame oil (or butter) over medium heat with 1 clove crushed garlic and the sesame seeds. Cook, stirring, until the sesame seeds are toasted and the garlic is fragrant, about 3–5 minutes; remove from heat.
Serve the noodles topped with extra green onions, chopped roasted peanuts, and the garlicky toasted sesame seeds. Optionally squeeze lime juice over each serving.
