Preheat the oven to 425 degrees F. Toss the mushrooms with the extra-virgin olive oil, garlic, sage, salt, and pepper, and roast 25 minutes.
Meanwhile, heat the pumpkin puree in a small pot over medium heat with a little chicken stock to thin it out. Season with salt, pepper, and nutmeg. Bring the remaining stock to a boil in a saucepot and whisk in the chipotle in adobo, its sauce, and polenta. Keep whisking the polenta until it pulls away from the sides of the pot, then stir in the butter and honey. Combine the pumpkin and polenta and serve in shallow bowls. Top with roasted mushrooms and garnish with pepitas and chives.