Heat oil in a heavy bottom pan over medium heat. Add leeks and peppers. Gently cook until soft, 6-8 minutes. Stir in curry paste and cook for another minute. Add in coconut milk, mirin, dark soy sauce, brown sugar, garlic, and ginger. Simmer for 10-12 minutes.
Meanwhile, bring beer to boil and separate pot. Add mussels. Cover and steam for 5-8 minutes until shells open. Drain. Add mussels to curry sauce. Garnish with cilantro.