Cover the cashews with water and let them soak for a few (2-3) hours (or overnight in the fridge). Then rinse and drain.
Cut the tempeh into chunks and toss them in a pan with oil over medium to high heat – add salt, cumin, and mustard seeds. Let them cook until golden-brownish. Then set aside.
Add the cashews, clean water, and salt into a blender. Blend until completely smooth.
In a pan over medium to high heat, cook the diced onion and minced ginger until soft.
Reduce the heat, then add the diced tomatoes and cashew sauce. Stir and add more salt to taste. If it's too thick, you can also thin it out with water and let it cook until warm. Stir in half of the tempeh cubes.
Add the other half of the tempeh cubes on top when serving. Enjoy with basmati rice and naan. Cheers!