Heat olive oil in a large skillet over medium-high heat; sauté spinach for 3 minutes until wilted. Season with salt, pepper, and nutmeg. Arrange toasted sourdough bread on 4 plates; divide spinach among toast slices and set aside. Bring a high-sided skillet filled with 2 inches of water to a simmer; add vinegar. Working one at a time, break each eggs into a ramekin and slide into simmering water. Gently poach 3 minutes; remove with a slotted spoon. Place 1 egg on top of each spinach toast (reserve last egg). In a blender, combine yolk from remaining poached egg (discard white) with mayonnaise, lemon juice, and water; blend until smooth. Add melted butter; blend to combine. Spoon hollandaise over eggs and serve immediately.
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