In a medium saucepan over medium-high heat, fill 3/4 of the pot with water and bring it to a boil. Add cauliflower and cook for 5-7 minutes, or until soft. Drain well when done.
When cool, in a food processor or high-speed blender puree the cauliflower with cannellini beans, onion, garlic, vegetable broth and ground pepper until almost smooth. Set aside.
Turn the grill on to medium-high and lightly grease the grill grates with either a 1/4 teaspoon of olive oil or cooking spray.
Remove the gills, which are the inside of the mushroom using a spoon and then wipe off the mushroom with a damp paper towel.
Using a basting brush, lightly grease the portobello mushrooms with 1 3/4 teaspoon olive oil, salt, and pepper. Grill for 5 minutes on each side until golden brown. When cool, slice portobello mushrooms on a diagonal and set aside.
In a small pot, add 1 teaspoon olive oil, onion, and minced garlic, and saute for about 3 minutes. Add red wine and balsamic vinegar; stir well about 10 minutes and set aside.
Place the cauliflower mixture on a platter. Add the portobello mushrooms on top and drizzle with the red wine sauce.
Serving size: 2 portobello mushrooms + 2 cups mashed cauliflower, Calories: 400 calories, Fat: 10 grams, Saturated fat: 1 gram, Carbohydrates: 55 grams, Sugar: 15 grams, Sodium: 385 milligrams, Fiber: 15 grams, Protein: 18 grams, Cholesterol: 0 milligrams