In a very hot stock pot, add 1 tablespoon oil, add clams and stir for 2 minutes. Add lemon grass, ginger, chiles and onion and stir for 2 more minutes. Season. Deglaze with mirin and reduce liquid by 80 per cent. Add chicken stock and turn heat down to simmer. When clams are open, remove and continue slowly simmering the broth. Check for seasoning. Reduce broth by 20 per cent, which should take about 30 minutes, then strain. Bring broth back to a boil; add udon, spinach, clams and butter. Add lemon juice and serve in large bowls.