Halve the carrot lengthwise, then thinly slice. Thinly slice the celery. Cut the unpeeled sweet potato into 2cm chunks. Strip the thyme leaves, discarding the tough stems. Crush or finely chop the garlic. Cut the fish into 2cm chunks.
Step 2 — Cook vegetables
Preheat the oven to 200C, fan-forced. Heat 1 tbs olive oil in a medium saucepan over medium heat. Cook the carrot, celery and thyme, stirring occasionally, for 7-8 mins until softened. Crumble the stock cubes into a heatproof jug, add 500ml (2 cups) boiling water and stir to dissolve.
Step 3 — Start chowder
Add 2 tsp olive oil to the vegetable mixture and stir to combine. Add 1½ tbs flour and cook, stirring, for 2-3 mins until foaming. Gradually add the stock and 125ml (½ cup) milk, stirring constantly to prevent lumps forming, then bring to a simmer.
Step 4 — Finish chowder
Add the sweet potato to the stock mixture. Reduce the heat to medium-low and cook, covered, for 10-12 mins until almost tender. Add the fish and prawns and cook, covered, for 4-5 mins until just cooked through. Stir in the spinach, half the fish sauce*, half the garlic and 2 tsp dijon mustard. Taste, then season with salt and pepper. Remove from the heat.
Step 5 — Warm rolls
While the fish and prawns are cooking, put the wholemeal rolls on an oven tray and bake for 4-5 mins until heated through.
Step 6 — Get ready to serve
Meanwhile, melt 40g butter in a small saucepan over medium heat. Add half the Spanish seasoning* and the remaining garlic and cook, stirring, for 1-2 mins until fragrant. Remove from the heat and season with salt and pepper. Divide the chowder among bowls. Drizzle with the smoky garlic butter and serve with the rolls.