Soak cashews in cold water for at least 2 hours or up to overnight. If you don't have time for this step, soak cashews in boiling water for 20 minutes.
Add soaked and drained cashews and diced tomatoes to a blender with all the seasonings for the sauce. Blend until very smooth, about 1-2 minutes in a high speed blender, or a bit longer in a standard blender. Set aside.