Bruschetta With Grilled Portobello Mushrooms

1A. FOR A CHARCOAL GRILL: Open bottom vent completely. Light large chimney starter filled with charcoal briquettes (6 quarts). When top coals are partially covered with ash, pour evenly over grill. Set cooking grate in place, cover, and open lid vent completely. Heat grill until hot, about 5 minutes.
1B. FOR A GAS GRILL: Turn all burners to high, cover, and heat grill until hot, about 15 minutes. Turn all burners to medium. (Adjust burners as needed to maintain grill temperature around 350 degrees.)
2. While grill heats, brush toasts with 2-1⁄2 tablespoons oil. Place mushroom caps on large baking sheet. Mix remaining 3-1⁄2 tablespoons oil, rosemary, and salt and pepper to taste in small bowl. Brush oil mixture over both sides of mushrooms.
3. Clean and oil cooking grate. Grill mushrooms, gill side up, until caps are cooked through and grill-marked, 8 to 10 minutes.
4. Halve grilled mushrooms. Place 1 half, gill side down, on each toast. Serve immediately.