Heat olive oil over medium-high heat in a wok or Dutch oven. After 90 seconds, when the oil is hot, add the diced onion and oyster mushrooms. Sauté for 4–6 minutes until lightly golden, stirring occasionally.
Stir in the minced garlic and grated ginger, cooking for 1 minute until fragrant.
Add the carrots, potatoes, grated apple, vegetable stock (or water), tamari, tomato paste and sugar. Stir to combine and bring the mixture to a boil. Reduce the heat to low and simmer, covered, for 18-20 minutes, or until the carrots and potatoes are tender.
While the vegetables cook, prepare the curry roux. In a small saucepan, melt the vegan butter over medium heat. Sprinkle in the flour and whisk constantly until a smooth paste forms. Cook for 2–3 minutes, whisking continuously, to remove the raw flour taste. Add the curry powder, garam masala, ichimi togarashi (if using), and whisk until evenly combined. Remove from heat.
Once the vegetables are tender, add all of the curry roux to the pot, and stir to thicken the curry, cooking uncovered at medium heat for another few minutes if you want the curry to thicken more.
Serve the curry over cooked short-grain rice. If desired, garnish with sliced scallions and tofu katsu.