Preheat the oven to 400°F (200°C).
Peel the celeriac and cut it into thick slices, resembling steaks.
Place the celeriac steaks on a baking sheet and brush both sides with olive oil. Season with salt and pepper.
Roast the celeriac steaks in the preheated oven for about 20-25 minutes, or until they are tender and golden brown.
While the celeriac steaks are roasting, prepare the miso sauce. Whisk together the miso paste, soy sauce, maple syrup, rice vinegar, grated ginger, minced garlic, and water in a small mixing bowl until well combined.
Heat a small saucepan over medium heat and pour the miso sauce mixture into it. Cook for 3-4 minutes, stirring occasionally, until the sauce thickens slightly.
Remove the celeriac steaks from the oven and serve them hot, drizzled with the miso sauce.