In a large saucepan, place 3 litres of water and bay leaves, season to taste with sea salt and bring to the boil, then reduce heat to a gentle simmer. add baby fennel and cook for 4 minutes. Add brussels sprouts and cook for another 4 minutes. Add cauliflower and cook for 5 minutes or until vegetables are just tender. Drain and place on a large platter and season to taste with sea salt and freshly ground black pepper.
For mustard dressing, place lemon juice, olive oil, reserved fennel fronds and mustard in a small screw-top jar and shake until combined. Pour dressing over vegetables, then scatter with feta and pinenuts.