Prepare the vinaigrette; Feed apple through a juicer. Stir in vitamin C powder. Using a spatula or plastic bowl scraper, push currants through a tamis, a very fine mesh strainer. Combine currant puree and 3 tablespoons apple juice in a medium bowl. Gradually whisk in olive oil until emulsified; season with salt and pepper. Whisk in porcini oil. Vinaigrette can be stored, refrigerated in an airtight container, up to 1 week.
Prepare the tuna tartare; Season tuna with salt and pepper. Divide between 2 plates. Drizzle with olive oil; lightly sprinkle vinaigrette on top. Garnish with frisee and radish. Serve immediately.