Preheat the oven to 200°C/gas mark 6. Toss the sweet potatoes with the oil and season. Place on a large baking tray and bake in the oven for 50-55 minutes, until crisp, tossing twice throughout the cooking time.
Meanwhile, spread the breadcrumbs on a large baking sheet and place in the oven for 15 minutes, tossing once halfway through, until golden and crisp. Set aside.
Cut each salmon fillet in half lengthways and widthways, so you end up with 16 even pieces.
Place the flour and mustard in two small bowls. Dip each salmon piece into the flour, then coat it in the mustard. Finally, dip it into the breadcrumbs and place on a baking sheet. Repeat with the remaining salmon pieces. Place them in the oven for 10-15 minutes, until just cooked through.
Meanwhile, combine the yogurt with the juice of half the lemon and some seasoning. Combine the bistro salad, cherry tomatoes and cucumber in a bowl and toss with some of the sauce. Cut the remaining half of the lemon into wedges.
Divide the fish fingers and sweet potato chips between 4 plates. Serve with the salad, remaining sauce and lemon wedges.