Position oven rack 6 inches from broiler. Preheat broiler. Mix first 4 ingredients in bowl. Place mushrooms, stem side up, on heavy large baking sheet. Brush with oil mixture. Season with salt and pepper. Broil mushrooms until just cooked through, about 3 minutes. Place on work surface. Set oven temperature at 450°F.
Spread Fontina cheese strips on bread. Cut each mushroom crosswise into 1/2-inch-wide strips atop each bread slice. Place bread on baking sheet. Bake until cheese melts, about 5 minutes.