Preheat the oven or toaster oven to 375 degrees F. Spread cashews on a baking tray and bake for 3 minutes. Gently stir cashews to roast them evenly. Return to the oven for another 3 minutes, or until golden brown. Watch carefully because cashews can burn quickly. Remove from the oven and cool for at least 30 minutes.
Cut the tougher, bottom stems (about 1-1/2 inches) off the spinach. If you don't like to eat the stems, cut off all of them but add more spinach leaves to compensate. Combine spinach and arugula (or rapini) in a large bowl.
Heat oil in a large frying pan on medium-high heat for 1 minute. Add cumin seeds, stir and allow to sizzle for 30 to 45 seconds. Add tomatoes, mustard seeds, turmeric, salt and cayenne. Stir and reduce the heat to low, then cover and cook for 5 minutes. Remove the lid and stir. The top of the masala should glisten with oil. If not, cook uncovered for another 1 to 2 minutes to make sure the spices are cooked through.
Stir in water and increase the heat to medium. Bring to a boil, then reduce the heat to low and cook at a low boil for 5 to 8 minutes. Add coconut milk, increase the heat to medium and continue cooking until the curry starts to boil. Add greens and cook them for 2 minutes, stirring regularly.
Place 2 slices of paneer in each of six large bowls. Pour the curry and green leaves over the paneer. Top the greens with 6 to 7 roasted cashews per bowl.