Bring a large pot of salted water to a boil. Add the rice noodles and cook until almost tender, about 3 minutes. Drain and rinse under cold water to cool. Drain the noodles very well, tossing with tongs to release as much water as possible. Snip the noodles with scissors a few times to cut the long strands.
Bring the chicken broth and 2 1/2 cups water to a simmer in a medium saucepan. Whisk in the peanut butter, soy sauce and Sriracha. Then add the chicken and bell pepper. Cook, over medium-low heat, at a bare simmer until the chicken is just cooked through, 3 to 5 minutes. Add the cooked rice noodles and scallions to the soup. Divide among 4 bowls and top with the cilantro, cucumbers, romaine and serve with lime wedges.
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