Shake off excess water from the spinach, put in a large saucepan over medium heat with only the water clinging to the leaves. Sprinkle with a little salt. Cover and cook for 2 to 3 minutes, or until wilted. Drain, pressing down excess liquid, then place in a greased ovenproof dish.
In a small saucepan, heat 1 tablespoon butter over medium heat, add the mushrooms and cook for 2 minutes, stirring frequently. Add the pine nuts and green onions and cook for 2 minutes. Season with salt and pepper. Spread over the spinach.
Fill a skillet with 1½ inch water and bring to a boil. Add 2 tablespoons of vinegar. Reduce the heat to a gentle simmer. Carefully break an egg into the water. Add the remaining eggs and cook for 4-5 minutes, or until set. Carefully remove with a slotted spoon. Arrange the eggs on top of the spinach mixture.
Melt the remaining butter in a saucepan and stir in the flour. Cook for 2 minutes, then remove from heat and slowly add the milk and stir. Return to the heat and cook, stirring continuously until the mixture thickened. Add the mustard and ½ cup of the cheese. Continue stirring until the cheese has melted. Season with salt and pepper. Pour the mixture over the eggs, completely covering them. Sprinkle with the remaining cheese.
Bake for 20 to 25 minutes until hot and bubbly. Serve
Recipe by The Gluten Free at https://www.theglutenfree.com/gluten-free-eggs-florentine/