Mustard-Baked Salmon with Potato and Fennel Tzatziki

Ingredients

• 500 gm small kipfler potatoes, scrubbed, halved lengthways
• 60 ml (¼ cup) extra-virgin olive oil, plus extra to drizzle
• 2 garlic cloves, crushed
• 2 tbsp honey Dijon mustard
• 800 gm piece salmon, skin on, pin-boned
• 3 small lemons, thinly sliced, patted dry
• 1 baby fennel (180gm), coarsely grated
• 50 gm feta, coarsely grated
• 140 gm Greek-style yoghurt
• 2 tbsp lemon juice
• 1 tbsp chopped dill
• Watercress sprigs, to serve