01 - Set the oven to 400°F and allow it to reach temperature.
02 - Toss the celeriac cubes and garlic cloves with 1 tablespoon olive oil, 1/2 teaspoon sea salt, and black pepper.
03 - Spread the seasoned vegetables evenly on a parchment-lined baking sheet.
04 - Roast for 35–40 minutes, turning once halfway through, until golden and tender.
05 - Transfer the roasted celeriac and garlic to a food processor or bowl for mashing.
06 - Add unsalted butter, milk, and nutmeg if using; process or mash until smooth and creamy.
07 - Adjust salt and pepper as needed, blending or mashing again if necessary.
08 - Plate the mash and garnish with chopped chives or parsley as desired.