- In a blender or small food processor, purée the avocado with the lemon juice and mustard until smooth. Season with salt and pepper. Drizzle in the oil and blend until smooth and creamy. Adjust the seasoning.
- Spread each slice of bread with the avocado mixture. Arrange the tomato slices on half of the bread slices. Season with salt and pepper. Top with arugula. Close with the remaining slices of bread. Serve immediately.