Cut the unpeeled sweet potato into 1.5 cm chunks. Finely chop half the onion and thinly slice the remaining half. Drain and rinse the chickpeas. Put the sliced onion and 1 tsp white vinegar in a bowl and toss to coat. Set aside to pickle, tossing occasionally, until needed.
Step 2 — Start curry
Heat 2 tsp vegetable oil in a medium deep frypan over medium heat. Cook the chopped onion, stirring, for 2-3 mins until softened. Add the curry powder, Indian butter curry paste and tomato paste and cook, stirring, for 1 min or until fragrant.
Step 3 — Simmer curry
Add the sweet potato, chickpeas and 375ml (1½ cups) water to the onion mixture. Increase the heat to medium-high and cook, covered, for 14 mins or until the sweet potato is tender. Increase the heat to high, add the peas and cook, covered, for 2 mins or until tender. Remove the pan from the heat. Taste, then season with salt and pepper.
Step 4 — Toast bread rolls
Meanwhile, preheat the grill to high. Cut the bread rolls in half. Put on an oven tray, cut-side up, and grill for 4-5 mins until lightly toasted.
Step 5 — Serve up
Drain the pickled onion, discarding the pickling liquid. Spread the bread roll bases with 2 tbs vegan mayonnaise, then fill with the curry and pickled onion. Sandwich with the tops and enjoy!
Step 6 — Make it yours
This is a saucy curry so eating it in a roll can be a little messy – yet so delicious! If you prefer, simply serve the bread alongside instead.