Preheat oven to 200°C (180°C fan).
Heat olive oil in a large ovenproof pan over high heat. Add garlic and fennel, cooking for 1–2 minutes until fragrant.
Add peas, stock, lentils, and saffron if using. Season and stir well. Bring to a boil, then remove from heat.
Season the salmon and place it skin-side down over the lentil mixture.
Bake for 10–15 minutes until the salmon is cooked to your liking. This One Pan Salmon with Fennel and Lentils should have a tender, slightly pink center for best texture.
Rest for 10 minutes. Scatter parsley and lemon zest on top before serving with lemon halves.