Pulse the toasted bread in a food processor until crumbs - you want them to be reasonably fine. Transfer to a large bowl and set aside.
Put potatoes in a medium saucepan and cover with water. Bring to a boil, then reduce heat and simmer for 10-15 minutes, until tender. Remove from heat and drain. Allow to cool before peeling and discarding the skins. Transfer potatoes to a food processor and set aside.
Meanwhile, heat 1 tbsp oil in a frying pan over medium-high heat. Add spring onion and garlic and cook for 3-5 minutes, stirring often, until fragrant and golden brown. Add to the food processor with the potatoes and blitz until smooth.
Add to the bowl with the breadcrumbs, and add trout, egg, lemon zest and salt. Mix to fully combine. Cover and refrigerate for at least 30 minutes.
Using well-oiled hands, form 6 patties from the trout mixture.
Tip the panko breadcrumbs into a shallow bowl and gently press one trout pattie into the crumbs on all sides. The coating of oil from shaping the patties should be enough to ensure the breadcrumbs stick. Repeat with remaining patties.
Heat the remaining 2 tbsp olive oil in a large frying pan over medium-high heat. Cook patties for 4-5 minutes on each side, until nicely browned and crisp. You may need to cook in 2 batches to avoid overcrowding the pan. Add more oil if needed.
Serve hot, with the Saffron, Mustard Seed & Tarragon Aioli on the side.
Crush the saffron threads with your fingers to bruise, and place in a small bowl. Cover with 1 tbsp freshly boiled water and leave to steep for 10 minutes. Remove the saffron threads and discard or save for another use.
Meanwhile, set a small dry pan over medium heat and add mustard seeds. Just when they begin to pop, reduce heat to medium-low and cover with a lid. Cook for 1 minute, shaking the pan occasionally, until fragrant (watch as they can burn easily). Transfer to a small bowl and set aside to cool.
Using a stick blender, blend egg yolks, lemon juice, garlic and mustard to combine. While blending, add a few drops of oil to emulsify, then increase to a thin stream. It should take 2-3 minutes to add all of the oil - this will prevent it splitting and ensure a thick and creamy texture. Stir through maple syrup and salt, along with the saffron liquid, cooled mustard seeds and dried tarragon. Store in the fridge until ready to use.