Heat the olive oil over medium heat in a large (12-inch) sauté pan. Add the leeks and sauté for 3 minutes, until tender. Add the prosciutto and sauté for 2 to 3 minutes, breaking it up with a fork. Add the spinach, tossing with tongs, then cover the pan and cook for 3 minutes, tossing once while it cooks, until the spinach is all wilted. Off the heat, stir in the basil and lemon juice and set aside for 5 minutes. Divide the mixture with tongs among the 12 muffin cups, leaving any liquid behind. Sprinkle the Fontina evenly on top.