Steps
1. Preheat your oven to 350°F (175°C).
2. In a small bowl, melt the butter and let it cool slightly.
3. In a separate bowl, whisk together the egg, milk, mustard, sugar, and salt until well combined.
4. Stir in the melted butter and finely chopped bell pepper.
5. Pour the mixture into the pre-made tart shell.
6. Bake in the preheated oven for 20-25 minutes, or until the custard is set and the top is lightly golden.
7. Allow to cool before serving to enhance the flavors.