6 tablespoons (about) olive oil
1 teaspoon minced fresh rosemary
1 teaspoon minced fresh tarragon
4 very large portabella mushrooms, stemmed, dark gills removed, caps thinly sliced
8 1/2-inch-thick slices country white bread (each about 3x6 inches)
Dijon mustard (optional)
8 ounces Fontina cheese, thinly sliced
2 bunches arugula or 1 bunch trimmed watercress
Heat 4 tablespoons oil in heavy large skillet over medium-high heat. Add rosemary and tarragon stir 30 seconds. Add mushrooms. Cover skillet and cook until mushrooms are tender, stirring often, about 8 minutes. Season mushrooms to taste with salt and pepper; transfer to plate. Wipe out skillet.
Arrange 4 bread slices on work surface; spread with mustard, if desired. Top bread slices with cheese, then mushrooms and arugula, dividing equally. Top each with bread slice; press to compact. Brush tops lightly with oil. Heat reserved skillet over medium heat. Place sandwiches, oiled side down, in skillet. Brush tops lightly with oil. Cover and cook until golden on bottom, occasionally pressing with spatula, about 5 minutes. Turn sandwiches over. Cover and cook until golden on bottom and cheese is melted, about 5 minutes longer. Transfer sandwiches to plates; cut in half and serve.
Makes 4 Servings.