In a mixing bowl, whisk together eggs, salt and pepper.
Sauté onions in olive oil in wok or sauté pan, until translucent. Add in garlic and kale, continue cooking for a few more minutes. Add in aji mirin and tamari and cook until soft.
In cast iron pan, melt butter or olive oil over medium heat. Pour in mixed eggs. Add kale, herbs and feta cheese. Place pan in oven. Cook for ten minutes until eggs are solid.
Cut in half and serve immediately.
Recipe by iEatGreen at https://www.ieatgreen.com/recipe-kale-frittata-2/