Heat oil in a large skillet and add the mustard seeds to it when it gets hot.
When the mustard seeds splutter, add the cashews and let is roast till it starts changing color.
Add the sliced onion, a little salt for faster cooking and saute for a while.
When the onion starts changing color to light brown, add the crushed ginger-garlic- fennel mixture and mix well.
Once their raw smell disappears, add sliced leaves: 2-3 mint or 2 tsp of coriander leaves.
Mix well and add the turmeric powder and continue sauteing.
Add the tomatoes, a little salt and continue cooking till the tomatoes turn soft and is cooked well.
Add the red chili powder and garam masala and let it all cook till oil starts coming out and masala starts leaving the sides of the pan.