1. To make the marinade: In small bowl, whisk together first 7 ingredients.
2. Remove and discard the mushroom stems. Scrape out the dark gills with a teaspoon. Generously brush the mushrooms all over with some of the marinade and reserve the rest. Allow the mushrooms to marinate at room temperature for 30 minutes.
3. Grill the bell peppers over Direct High heat until the skins are blackened and blistered all over 12-15 minutes, turning ocassionally. Place the peppers in a small bowl and cover with plastic wrap to trap the steam. Set aside for at least 10 minutes, then peel the skin from the peppers, discarding the stems and seeds, and cut them into 1 inch squares. Place the roasted peppers in a medium bowl.
4. Grill the mushrooms over direct medium heat until tender 12-15 minutes, turning occasionally and if neccesary to prevent drying out, brushing with the reserved marinade. Cut the mushrooms into 1-inch pieces. Add them, along with the parsley and capers to the bowl of roasted peppers. Add the reserved marinade. Toss to distribute th ingredients. Cover and set aside at room temperature or at least 1 hour. Taste just before serving and, if necessary, season with salt and pepper. Serve at room temperature.