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Heat grill to high and lightly toast the bread on both sides. Spread 1 tsp mustard over one side, then top with the ham and cheese. Whisk the remaining mustard and vinegar with 1 tsp oil and seasoning.
Grill the croque for 3 mins, or until the cheese is bubbling and turning golden. Meanwhile, heat a non-stick frying pan. Add a little oil, then crack in the egg and gently fry to your liking. When cooked, lift it on top of the toast. Toss the dressing with the spinach, and cornichons if using, then eat straight away.