To prepare the marinade, put all of the ingredients in a small bowl and whisk together. Remove the stems from the portobellos and discard them. Place the mushroom caps in a large bowl, pour the marinade over them, and toss well. Let marinate for at least 30 minutes.

To prepare the sauce, put the red wine, garlic, and shallots in a medium saucepan and reduce over high heat until about 1/3 cup of liquid remains, about 4 minutes. Add the vegetable stock and peppers, decrease the heat to medium, and reduce until about half of the liquid remains. Puree the sauce in the pan with a handheld blender. Alternatively, transfer the sauce to the bowl of a food processor and blend, then return to the pan. Season to taste with the coriander, cumin, and cilantro. Add the butter, and stir well. Season to taste with the salt and pepper. Keep warm.

To prepare the mushrooms, oil the grill racks and heat the grill to very hot. Remove the mushrooms from the marinade, letting the excess marinade drain off, and place the caps on the grill. Grill on each side for 3 to 4 minutes, or until tender.

To serve, divide the mashed potatoes or polenta among the serving plates. Top with the mushrooms and pour some of the sauce over them. Serve immediately with the remaining sauce in a bowl on the side.

Per serving: 303 Calories (kcal); 26g Total Fat; (80% calories from fat); 4g Protein; 11g Carbohydrate; 3mg Cholesterol; 33mg Sodium Food Exchanges: 0 Grain(Starch); 0 Lean Meat; 2 Vegetable; 0 Fruit; 5 Fat; 0 Other Carbohydrates