Before you start, preheat oven to 220°C (or 200°C fan bake).
Prep onion and spinach. Heat oil in a large fry-pan on high heat. Cook onion with a pinch of salt for about 4 minutes, until softened and browned. Add first measure of balsamic vinegar and honey and cook for a further 1 minute, until caramelised.
Add spinach to pan and cook for a further 2 minutes, stirring, until spinach has wilted. Transfer mixture to a medium bowl. Crumble feta into bowl, along with spices, breadcrumbs and egg yolk. Mix well, until combined.
Gently unroll pastry onto a flat surface. Shape filling into a log along centre of the pastry. Roll up pastry to form a long roll. Cut into 8 even-sized pieces and place on a lined oven tray, seam side down.
In a small bowl, combine all super seeds together. Brush rolls with milk, sprinkle over super seeds and season lightly with salt. Bake on middle oven rack for about 20 minutes, until golden and filling is cooked through.
Prep lettuce, cucumber and basil and toss in a large bowl with Parmesan and second measure of balsamic vinegar. Season to taste with salt and pepper.
Serve rolls with salad and beetroot relish.