Prepare the flat rice noodles as directed on the package until they are barely tender, then drain and rinse them under cold water. The noodles should bend easily while still feeling firm enough to resist a gentle bite.

Whisk the fish sauce, soy sauce, brown sugar, rice vinegar or tamarind paste, Sriracha, and optional peanut butter in a bowl until blended. The mixture should taste boldly sweet, salty, and tangy because the noodles and vegetables will mellow it in the pan.

Warm 1 1/2 tablespoons of oil in a wok or large saucepan over medium-high heat, then cook the garlic, bell pepper, and chosen protein. Shrimp should turn pink in about 1-2 minutes per side, while chicken needs about 3-4 minutes and should be cooked through after one turn.

Move the cooked ingredients to one side of the pan, add a little more oil to the empty space, and scramble the eggs there with a spatula. Break the eggs into small, soft curds and fold them in while they are still moist.

Add the noodles, sauce, bean sprouts, and peanuts, saving some peanuts for the finish, then toss briskly until combined. The sauce should bubble briefly and cling to the noodles in a glossy brown coating rather than pool at the bottom of the pan.

Finish with green onions, the reserved peanuts, cilantro, and lime wedges, then serve right away. The residual heat will wake up the peanuts and herbs without taking away their fresh crunch.

Cool any remaining noodles before transferring them to the refrigerator. Keep leftovers covered and enjoy them within 2-3 days.