Preheat the oven to 400°. Arrange the bread on a baking sheet and drizzle with 3 tablespoons of the olive oil. Toast for 12 minutes, until slightly golden around the edges. Turn the broiler on.
Meanwhile, thinly slice the mushrooms. In a large skillet, cook the butter over high heat until lightly browned, 2 minutes. Add the remaining 2 tablespoons of olive oil and the mushrooms and cook undisturbed until the mushrooms are browned on the bottom, 2 minutes. Continue to cook, stirring occasionally, until browned all over, 10 minutes. Add the shallots, garlic and thyme, lower the heat to moderate and cook until the shallots are tender, 5 minutes. Add the water and scrape up any browned bits from the bottom of the pan, then cook for 3 minutes longer. Season the mushrooms with salt and pepper and remove from the heat.
Spoon the mushroom mixture on the toasts and sprinkle the cheese on top. Broil for 2 minutes, or until the cheese is melted. Transfer the crostini to a platter, sprinkle with the parsley and serve.
The mushrooms and Fontina suggest a citrusy-lush Pinot Blanc, but an earthy-spicy Pinot Noir would be terrific as well.