How to Make It
Preheat the oven to 400°. In a medium roasting pan, drizzle the leeks, fennel and shiitake with olive oil and season with salt and pepper. Toss to coat. Roast for about 50 minutes, until the vegetables are tender and browned in spots.
Meanwhile, in a medium saucepan, bring the chicken stock to a boil. Slowly whisk in the polenta and cook over moderately low heat, whisking occasionally, until smooth, about 5 minutes. Remove from the heat and stir in the Parmigiano-Reggiano, butter and thyme. Season with salt and pepper. Spoon the polenta into shallow bowls, top with the vegetables and serve.