CHEESE SAUCE: Mix the cornstarch to a paste with about a 1/4 cup of the milk. Pour the remaining milk and butter into a nonstick saucepan and bring to a boil. Reduce to simmer.
Stir the boiling milk into the cornstarch mixture, then pour back into the pan. Bring to a boil, stirring. Once it has thickened, simmer for 2 minutes.
Remove from the heat, stir in the Gruyère cheese, salt, pepper, and optional nutmeg. Cover the surface of the sauce with parchment paper to prevent a skin from forming and set aside in a warm place.
SPINACH: Heat the oil in a large saucepan. Add the green onion (or leeks) and saute' for about 3 minutes, until they begin to soften.
Add the spinach and stir over medium heat for 2–3 minutes until the spinach has wilted and the leeks are tender.
Drain the vegetables in a sieve or fine mesh strainer, pressing down with the back of a spoon to remove any excess moisture. Return to the pan and season with dill, salt, and pepper. Cover and keep warm.
EGGS: half-fill a medium sized frying pan with water and bring to a simmer. Add the vinegar. Break in the eggs, and cook gently for 3–4 minutes, spooning the hot water over the yolks. Lift out the eggs with a slotted spoon and drain on paper towel.
Preheat the broiler. Place the whole wheat toast slices onto a parchment lined baking sheet. Spread the leek and spinach mixture evenly over each toast. Make a hollow in each spinach mixture using the back of a spoon and place a poached egg in each hollow.
Spoon the cheese sauce over the eggs. Lightly dust with paprika, then place the dish under the broiler. Broil until just lightly browned.