To begin, in a bowl, mix together mayonnaise, mustard, lemon juice, salt, pepper, garlic powder, red chilli flakes, and chopped onions. Mix well.
Drain the tuna chunks and roughly shred them. Add the tuna flakes to the dressing and mix well.
Take a 6” loaf of multigrain bread and warm it a bit. Slice the bread from the side but make sure you do not cut it all the way through.
Place a few lettuce leaves at the bottom.
Top it with a few spoonfuls of tuna salad.
Finally, place the sliced tomatoes on top. Lightly press down the sandwich.
The Multigrain Tuna Salad Sandwich is ready to serve. Serve it with fresh salad and potato wafers.