Add the plain yogurt, mint leaves, lemon juice, salt, and water to a blender or use an immersion blender.
Blend until creamy and speckled with mint.
Add more water, 1 teaspoon at a time, if the dressing is too thick. Taste and adjust with more lemon juice or salt if needed.
Slice the tomatoes into rounds and arrange them on a serving plate.
Scatter the very thinly sliced red onion over the tomatoes.
Spoon the mint yogurt dressing over the tomatoes and onions.
Sprinkle with a small pinch of garam masala, if using.
Garnish with extra mint leaves and serve right away.