In a bowl, mix kosher salt and sugar. Create a small bed of the salt-sugar mixture in a shallow dish. Gently place egg yolks on top, then cover completely with the remaining mixture. Refrigerate for 3-4 days until firm.

Make the Tuna Tartare

Prepare the Tuna

Dice the fresh sushi-grade tuna into small cubes. In a bowl, combine tuna with soy sauce, sesame oil, lime juice, and chopped green onions. Season with salt to taste. Set aside.

Prepare the Avocado

Slice the avocado into thin slices. Set aside for assembly.

Make the Spicy Potato Aioli

Blend the Aioli

In a blender, combine mayonnaise, cooked potatoes, sriracha sauce, minced garlic, lemon juice, and salt. Blend until smooth and creamy. Adjust seasoning to taste.

Assemble the Tacos

Build the Tacos

Warm the small tortillas in a pan. Spread a layer of spicy potato aioli on each tortilla. Add a generous portion of tuna tartare, and top with avocado slices. Finally, grate the cured egg yolk over the top before serving.