Preheat the oven to 180°C and grease a small oven-proof pie dish.

In a saucepan, gently melt the butter and stir in the flour.

Gradually add the warm milk, whisking until smooth. Bring to a boil and continue stirring until the mixture thickens. Add the mustard, Gruyere and half the parmesan and stir until the cheese has melted.

In a separate bowl, whisk the eggs until fluffy.

While whisking continuously, slowly add the eggs to the milk mixture and mix well to combine. Season with Sea Salt and Black Pepper to taste and pour the mixture into the prepared pie dish.

Sprinkle with the remaining parmesan and bake in a hot oven for 20-25 minutes until the custard is just set and golden brown.

Serve immediately with hot buttered toast.