Celeriac:

1. Preheat the oven to 200°C (400°F).
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2. Peel the celeriac and cut it into steaks. Score the steaks lightly with a knife.
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3. In a small bowl, mix together olive oil, mustard, maple syrup, soy sauce, salt, and pepper.
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4. Brush the mixture onto the celeriac steaks.
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5. Place the celeriac steaks on a baking tray and bake in the preheated oven for 25 minutes. Flip the steaks and bake for another 20 minutes or until tender and golden.
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Sauce:

1. In a saucepan, melt vegan butter over medium heat.
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2. Add diced onion and minced garlic. Sauté until softened.
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3. Stir in mustard, miso paste, curry powder, dried thyme, dried rosemary, salt, pepper, and chopped parsley. Cook for a few minutes.
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4. Pour in vegan cream, lemon juice, and agave. Let the sauce simmer for 5 minutes, stirring occasionally.
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5. Serve the sauce over the baked celeriac steaks.