In a heavy bottom saucepot, bring water to a boil. Stir in pearl barely, steel cut oats, and salt. Cover pot and remove from heat. Let pot sit covered at room temperature overnight.
The following morning, set pot over medium heat. Uncover and bring to a boil. Reduce heat to a simmer and cook for 5 minutes, stirring often, until mixture is thick and creamy. If need, add additional water to achieve desired consistency.
Remove from heat. Stir in miso paste until well distributed. Add more miso paste according to taste.
Divide oatmeal between two bowls. Serve immediately with fried egg, a drizzle of toasted sesame oil, chopped green onions, and season with freshly ground black pepper.