Curry powders all vary in their levels of heat, so if you're not sure about how much to use, start with 1 tablespoon, then, after adding the water, add more a bit at a time until you're happy with its spice.
Pour half of the coconut milk into a large skillet over medium-high heat. Whisk in the curry powder (see note) and salt, and whisk out any lumps. Add the red onion and garlic and cook for a minute. Stir in the rest of the coconut milk and the water, the add the tofu (if using). Cook, stirring occasionally for 2-3 minutes, then add the green beans and cauliflower.Cover and simmer 1 or 2 minutes, just until the cauliflower and green beans cook through.
Remove from heat and stir in the cashews. Taste and adjust the seasoning. Serve with topped with chopped cilantro.