For the Bolognese sauce
- 2 tablespoons peanut oil
- 1 onion, peeled and diced
- 1 carrot, peeled and diced
- 1 stalk celery, peeled and diced
- Kosher salt and freshly ground black pepper
- 1 clove garlic, peeled and minced
- 8 to 12 ounces mixed wild and cultivated mushrooms, cleaned, trimmed, and diced
- 1 tablespoon fresh thyme leaves
- 1 tomato, seeded and diced
- 1 cup brown chicken stock or canned reduced-sodium chicken broth
For the polenta
- Kosher salt
- 1 cup polenta (yellow cornmeal)
- 1/4 cup extra virgin olive oil
- 4 tablespoons freshly grated Parmigiano-Reggiano cheese
1. To prepare the Bolognese sauce: Heat the oil in a large skillet over medium heat until it moves easily across the pan. Add the onion, carrot, celery, salt, and pepper, and cook, stirring occasionally, until the vegetables begin to soften, about 5 minutes. Add the garlic, cook for 1 minute, then add the mushrooms and thyme leaves. Cook, stirring frequently, until the mushrooms are almost tender, about 3 minutes. Add the tomato, cook about 2 minutes more, then add the stock, 2 tablespoons at a time, bringing the pan to a simmer before each addition. Simmer the Bolognese until it is concentrated but not yet dry, about 30 minutes. Set aside to cool.
2. To make the polenta: Bring 4 cups of water to a boil in a saucepan over high heat. Add a pinch of salt and gradually whisk in the polenta. Stirring constantly, bring the polenta to a boil, then adjust the heat to low. Cook the polenta, stirring occasionally, until it is no longer grainy, about 30 minutes. Whisk the oil and salt to taste into the polenta and remove it from the heat.
3. Assemble the gratin: Preheat the oven to 350°F. Spoon half the polenta into a medium baking dish (an 11-inch oval dish works fine) and cover with half of the sauce. Spoon in the remaining polenta, spread it evenly, then sprinkle with the cheese. Transfer the remaining sauce to a small saucepan and reserve.
4. Bake the gratin until the top is golden, about 40 minutes. Just before serving, warm the reserved sauce over low heat. Divide the gratin and sauce among 4 plates, top each serving with sauce, and serve.