How to Make It
Put the stock in a medium saucepan and bring to a simmer. In another medium saucepan, heat 1 tablespoon of the butter over moderately low heat. Add the shallot. Cook, stirring occasionally, until translucent, about 3 minutes. Add the rice. Stir until the rice begins to turn opaque, about 2 minutes.
Add about 1/2 cup of the simmering stock and 1 teaspoon of the salt and cook, stirring constantly, until the stock has been completely absorbed. The rice and stock should bubble gently; adjust the heat as needed. Continue cooking the rice, adding the stock 1/2 cup at a time, allowing the rice to completely absorb the stock before adding the next batch. Cook the rice in this way until tender, about 25 minutes in all. You may not need to use all of the stock. Stir in the wine and simmer 1 minute.
Stir the Parmesan and 1/8 teaspoon pepper into the rice. Transfer to a shallow dish and let cool. Cover and refrigerate for at least 4 hours. Divide the rice into six portions. With your hands, shape each portion into a cake, about 3 inches in diameter and 3/4 inch high.
Heat the oven to 375°. Spread the pancetta on a baking sheet and bake until crisp, stirring once, about 15 minutes.
If using shiitakes, remove the stems. Slice all the mushrooms. In a medium frying pan, heat 2 tablespoons of the butter over moderate heat. Add the sliced mushrooms and cook, stirring occasionally, until they begin to brown, about 5 minutes. Add the garlic, the remaining 1/2 teaspoon salt and a pinch of pepper and cook 1 minute longer. Stir in the tomato and cover to keep warm.
In a large nonstick frying pan, heat the oil with the remaining 1 tablespoon butter over moderately high heat. Add the risotto cakes and cook until brown on both sides and hot in the center, about 3 minutes in all. You may need to cook the cakes in two batches.
Divide the mushrooms among six plates. Top each pile of mushrooms with a risotto cake and sprinkle with the pancetta.