In a stockpot over medium-low heat, combine peanut butter, soy sauce, ginger, maple syrup, garlic, and 1 cup water, stirring constantly, until mixture is thoroughly heated, 5 to 7 minutes.
COOK NOODLES
Meanwhile, cook noodles according to package instructions. Drain well in a colander, return to pot, and cover to keep warm.
FINISH PEANUT SAUCE
Add frozen vegetables and pepper flakes (if using) to peanut sauce; cook until vegetables are heated through, about 5 minutes. Season with salt to taste.
ASSEMBLE & SERVE
Add peanut sauce and vegetables to noodles; using tongs, toss well to coat. Top with scallions and serve immediately.