Onion salad goes wet and bitter
Cucumber &lt;&lt; water, water, water +++ 
Tomatoes &lt;&lt; water, water 
Onion &lt;&lt; water


Do you see how much water each of those main ingredients contain?

Let's take one at a time.

Onions:-

Fresh onions are not only have a high volume of water, also natrual chemicals that make strong pungent flavour. These chemicals (e.g. Sulphur) work quite well with oxygen in the air, resulting the bitterness.

In order to tone down the flavour and bitterness, soack the onions in ice water.
Another method is to add fresh lime juice immediately after slicing/dicing the onions.

Cucumber:-


Cut and remove the entire section of seeds. Now dice the cucumber. 
Remember the little science projects about drawing out excess water from vegetables? 
Soak Cucumber in ice water with little salt. (Perhaps you could refrain from adding salt to the final salad bowl, if the quantity used at this point suffice)


Same theory goes for making potato chips. Slice potatoes are soaked in water to draw out that extra moisture. No ricket science here though ;)

Tomatoes:-
Remove entire section of seeds. Dice them and add into your salad.

Garam masala has Fennel and Cineman, that has the sweetness. It's also possible restaurant adds in a bit of sugar. However, certain onion species are sweeter naturally.

To be frank, we don't use garam masala, mint or corriander. Insated you could have the following simple ingridients to make a heaven of a salad, if you may.

fresh :{cucumber, tomatoes, onion, spicy green chilli, black pepper, salt, lime}

Bon appetit!