Preheat the oven to 200°C (400°F).
Wash and dry the potatoes. For faster cooking and crispier edges, cut them in half lengthwise. Place them on a baking tray, drizzle with oil, sprinkle with salt, and toss to coat. Arrange the potatoes cut-side down for the best texture.
Bake for about 30–40 minutes if halved (or 40–50 minutes if left whole), until the potatoes are golden and tender throughout.
Remove from the oven and let cool slightly.
Cut each potato open lengthwise (if not cut before baking) and gently fluff the inside with a fork.
Top each potato with a generous spoonful of vegan tuna salad.
Sprinkle with grated cheese while the potatoes are still warm.
Finish with cucumber, cherry tomatoes, arugula, and red onion.
Serve immediately.