In a large pot over high heat, bring water to boil for noodles.
In a large straight-sided frying pan over medium-high heat, whisk together all ingredients except fish and noodles. Bring to boil; reduce to simmer. Add tilapia fillets, cover, and poach for 10 minutes.
Add udon noodles to rapidly boiling water and cook for 6 to 8 minutes for thin udon and 10 to 12 minutes for thicker udon. Serve udon in a bowl with miso broth. Top with poached tilapia and chopped scallions.