Preheat the oven to 220C, fan-forced. Thinly slice the onion. Finely chop the parsley, including the stems, keeping them separate. Crush or finely chop the garlic. Cut the unpeeled potato into 1.5cm chunks.
Step 2 — Start sauce
Heat 2 tsp olive oil in a medium deep frypan over medium heat. Cook the onion and parsley stems, stirring occasionally, for 3-4 mins until softened. Add the garlic and half the Italian seasoning and cook, stirring, for 30 secs or until fragrant.
Step 3 — Cook sauce
Add the potato, tomato passata, 2 tsp balsamic vinegar and 125ml (½ cup) water to the onion, cover and bring to the boil. Reduce the heat to medium and cook, stirring occasionally, for 15 mins or until the potato is tender.
Step 4 — Bake garlic bread
Meanwhile, put the garlic bread on an oven tray, butter-side up, and bake for 6 mins or until golden and crunchy.
Step 5 — Season prawns
Put the prawns and remaining Italian seasoning in a bowl, season lightly with salt and pepper and toss to coat.
Step 6 — Get ready to serve
Add the prawns to the sauce and cook, covered, for a further 2-3 mins until cooked through. Add the spinach and stir until wilted. Taste, then season with salt and pepper. Remove the pan from the heat. Crumble over the feta, then scatter with the olives and remaining parsley. Serve the prawns at the table with the garlic bread.