Separate scallion whites and greens, grate garlic and Parmesan, measure spinach, and beat eggs until no white streaks remain.
Heat olive oil over medium. Cook scallion whites 1 minute; add garlic and stir 20 to 30 seconds without browning.
Add rolled oats and stir 1 to 2 minutes, until nutty-smelling and slightly dry at the edges.
Add 2 cups broth, salt and pepper. Bring to a gentle boil, reduce heat and cook uncovered 4 minutes, stirring regularly.
Add 1/4 cup broth and simmer 1 to 2 minutes, until a spoon path across the bottom remains visible about 1 second.
At a gentle simmer, stir in one direction and pour beaten eggs in a thin stream. Cook 2 to 3 minutes until no liquid egg remains and the center reaches at least 160°F.
Fold in spinach in two additions until just wilted, about 1 minute. Add reserved broth as needed for a creamy flow.
Remove from heat. Stir in Parmesan, half the scallion greens and optional soy sauce after tasting. Adjust with broth by the tablespoon.
Divide into warm bowls and finish with remaining scallions, pepper, optional oil, lemon, chili crisp or sesame seeds.