How to Make It
Light a grill or heat a grill pan. Coat the mushroom caps with 3 tablespoons of the olive oil and season with salt and pepper. Grill the mushrooms, until deeply browned and crisp, about 5 minutes per side. Transfer to a large shallow dish to cool. Cut the caps in half, then thinly slice them crosswise; return to the dish.
Grill the peppers until charred all over. Transfer to a bowl, cover with plastic wrap and let steam for 5 minutes. Remove the charred skins, seeds and stems. Cut the peppers into 1/2 -inch strips, removing the ribs, and add to the mushrooms.
In a bowl, combine the remaining 3/4 cup of olive oil with the vinegar, 3 of the garlic cloves, the thyme, basil and 2 teaspoons of pepper. Pour all but 1/4 cup of the marinade over the mushrooms and peppers and season lightly with salt and pepper. Let marinate at room temperature for 1 to 3 hours. Drain before using.
In a small saucepan, boil the wine with the remaining 1 garlic clove until reduced to 2 tablespoons, about 7 minutes. Add the cream and bring to a boil. Remove the pan from the heat and stir in the shredded cheese. Whisk over moderate heat until the fondue is smooth. Season with salt and pepper.
In a large bowl, toss the spinach with the reserved 1/4 cup of marinade and mound on plates. Arrange the mushrooms, peppers and prosciutto alongside. Top with the sliced Fontina. Briefly reheat the fondue, whisking constantly until smooth. Drizzle the spinach with the fondue and serve the salad at once.