Using a teaspoon, gently scrape gills out of the bottom of the mushroom caps. Lightly rinse mushroom caps. Pat dry with paper towels. Cut tomatoes in half; remove seeds and stem.
Brush top and underside of the mushrooms and all sides of tomatoes with the 3 tablespoons olive oil. Sprinkle the mushrooms and tomatoes lightly with salt and pepper.
For a charcoal grill, grill mushroom caps and tomato halves, cut sides up, on the rack of an uncovered grill directly over medium coals for 6 to 8 minutes or until tender, turning once halfway through grilling. (For a gas grill, preheat grill. Reduce heat to medium. Place mushroom caps and tomato halves on the grill rack over heat. Cover and grill as above.) Remove from grill.
Cook pasta according to package directions. Drain; return to pot.
For dressing, coarsely chop the grilled tomatoes. In a blender or food processor combine the tomatoes, the 1/4 cup olive oil, lemon juice, the 1 teaspoon oregano, 1/2 teaspoon salt, and 1/2 teaspoon pepper. Cover and blend or process until smooth. Drizzle pasta with 2 tablespoons of the dressing. Keep warm.
To serve, place mushroom caps on serving plates. Divide pasta among mushrooms. Drizzle with remaining dressing. Sprinkle with additional snipped fresh herbs. Makes 6 servings.