In a medium bowl, combine all the sauce ingredients and mix well. Set aside.
In a large saucepan, heat the oil and add the watermelon cubes. Sauté for 6-7 minutes over high heat, stirring the watermelon occasionally. The watermelon should be lightly seared and wilted.
Add the sliced mushrooms, the beans and the sauce. Lower the heat to medium-low and cook for 10 more minutes.