Preheat oven to 180C. Heat oil and butter in a frying pan over medium-high heat, add leek and garlic and cook for 5 minutes or until very soft. Add spinach and cook another 2-3 minutes or until just wilted, then tip into a colander and press firmly using a wooden spoon to extract any excess liquid. Process in a food processor with 2 tbsp cream until finely chopped and season to taste with sea salt, freshly ground black pepper and nutmeg. Divide among four 1 cup-capacity ovenproof ramekins.
Crack an egg into each ramekin. Combine remaining cream and vinegar in a jug and stir to combine. Drizzle a little cream mixture over each egg, season to taste and bake for 20-25 minutes or until egg is cooked to your liking.
Meanwhile, for the Gruyère toasts, combine cheese, crème fraîche and mustard in a bowl, season to taste and mix to combine. Lightly toast bread under a hot grill, turn and spread with Gruyère mixture, return to grill and cook until cheese is golden and bubbling. Serve alongside baked eggs.