Mix together mayonnaise, lemon juice, salt, sugar and chilli flakes in a deep bowl. Start with 1 tsp lemon juice and 1 tsp sugar. Do a taste test and add more, if required. Sugar is added to balance the sourness of the lemon juice.
Add all the chopped veggies to the above dish and mix well.
Take each bread slice and spread the cheese spread. Place 1 -2 tbsp of the veg mixture on the bread slice and spread evenly. Cover with another bread slice with cheese side down. Repeat with the rest of the slices.
Notes
You can use eggless mayo for a complete veggie sandwich. Since the mayo already has salt in it, be careful while adding the salt. Also start with 1/2 cup of mayo initially and after adding veggies, add more if required.You can use grated carrots, purple cabbage ad just one type of capsicum for the sandwich. You can mix and match the veggies you like.You can also add chopped coriander and mint leaves if you like.If you like a grilled sandwich, you can apply butter on the outside and grill them as well. Store it in the fridge in an airtight container for up to 2 days. I usually cover the sandwich with a damp kitchen towel inside the container to prevent dryness.