To make the chilli and ginger dressing, heat the oil in a small saucepan over low heat. Add ginger and chilli, and cook, stirring, for 3 minutes or until soft. Stir in sugar and vinegar until sugar dissolves, then simmer for 5 minutes to mellow the vinegar taste. Add 2 tbsp water and bring to the boil. Remove from the heat. Add soy sauce, fish sauce and lime juice, and stir to combine. Set aside to cool.
Place sesame seeds on a plate and season generously with freshly ground black pepper. Press a piece of tuna into seed mixture to coat. Turn over and coat the other side. Repeat with remaining tuna. Heat oil in a large frying pan over medium–high heat. Cook tuna for 1½ minutes each side for medium–rare, or cook to your liking.
When ready to serve, heat a wok with the peanut oil on high and add the butternut spaghetti. Stir-fry for 2-3 minutes until al dente. Spoon over the dressing and toss to coat. Serve the butternut spaghetti in bowls topped with the sliced tuna, sliced avo, micro herbs, radish, pickled ginger and peanuts.