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Portobello Steaks with Mashed Celery Root

This vegan main course of portobello steaks with celeriac mash offers a sophisticated, earthy alternative to a traditional roast. The large, meaty portobello mushrooms are infused with a fragrant rosemary and mustard marinade before being roasted until tender and juicy. Paired with a velvety root vegetable purée, this dish highlights the natural savoury depth of plant-based ingredients while remaining light and nutritious.

Celeriac makes an excellent low-carbohydrate substitute for potatoes, providing a subtle, nutty flavour that is enhanced here by a touch of fiery horseradish. This recipe is perfect for a comforting weekend supper or an impressive dinner party option for vegan guests. Serve alongside garlicky seasonal greens or roasted asparagus for a complete, well-balanced meal that is as vibrant as it is satisfying.

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Ingredients for Portobello Steaks with Mashed Celery Root

  • 1/2 batch Rosemary Mustard Marinade

  • 2 large portobello mushrooms

  • 900g celery root

  • 2 tablespoons prepared horseradish

  • 3 tablespoons coconut butter

  • 2 teaspoons sea salt

  • 60 to 120ml unsweetened almond milk

  • 2 spring onions, thinly sliced

Preheat the oven to 350˚F.

Overnight, or several hours in advance, make the marinade and set it aside.

Remove the stems from the mushrooms, then use a fork to remove the gills. Do so by gently scraping in the direction of the gills about 1/8-inch deep. Set the mushrooms top side down in a baking dish and pour the marinade over them. Let them sit for at least 20 minutes or up to 8 hours.

While the mushrooms marinate, place the whole celery root into a glass baking dish with about 1/4 inch of water in the bottom. Cover the dish with tinfoil and roast the root in the oven for 45 minutes to 1 hour. Remove the tinfoil and allow the root to cool until you can handle it with your hands.

Use a knife to peel away the celery root skin, then roughly chop the root and place the pieces into a food processor. Purée the root with the horseradish and coconut butter, and add enough almond milk for the mixture to reach a mashed potato–like texture and consistency. Season with sea salt then fold in the chopped spring onions. Set aside and keep warm.

Place the mushrooms onto a baking dish and cook in the oven until tender, about 10 to 12 minutes.

Slice the mushrooms then set them on top of mounds of the celery root mash. Enjoy with garlicky greens or roasted asparagus.

Disclaimer

While every effort has been made to ensure the information is accurate and up to date, individual needs may vary and dietary requirements can differ based on personal health conditions. Always check food labels and allergen information before preparing or consuming any recipe. If you have specific health concerns, allergies, intolerances, or are following a medically prescribed diet, seek advice from your GP, pharmacist, or a registered dietitian before making significant changes to your diet or lifestyle.

Article history

The information on this page is peer reviewed by qualified clinicians.

  • 28 Jan 2026 | Originally published

    Authored by:

    UK recipe editors

    Peer reviewed by

    UK recipe editors
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