Place a wok or large non-stick frypan over medium-high heat.
Add 1 tsp oil and spinach. Cook till spinach wilts and turns dark green, about a minute. Transfer spinach to plate.
Add 1½ tsp oil to wok. Stir in onion and cook for 2 minutes. Stir in a pinch of salt. Add ginger and cook for 45 seconds.
Sprinkle chicken with remaining salt and add to wok with remaining 1½ tsp oil. Cook until chicken is opaque, 2 to 3 minutes. Reduce heat to low.
In a small bowl, whisk together stock, soy sauce and mirin; add to wok. Stir in spinach and cook until mixture begins to bubble gently. Pour eggs over top of chicken-spinach mixture; cover and cook until eggs set, 2 to 3 minutes.
To serve, spoon half a cup of rice into each deep soup bowl. Top with chicken-egg mix.Wanna take it to work? Stacked lunchboxes that keep different types of food separate (say rice, donburi and miso soup) are popular in Asia. Find 'em in Asian supermarkets, or check out the Tiffin Carriers at cinnamondesigns.com.au, which start at $35.