Before starting, preheat the oven to 450°F. Wash and dry all produce. Strip rosemary leaves from stem, then finely chop 1 tsp (2 tsp). Cut sweet potatoes into 1/2-inch wedges. Add sweet potatoes, rosemary and 1 tbsp oil to a parchment-lined baking sheet. (NOTE: For 4 ppl, use 2 baking sheets, with 1 tbsp oil per sheet.) Season with salt and pepper, then toss to coat. Roast in the middle of the oven, flipping halfway through, until tender and golden-brown, 22-24 min. (NOTE: For 4 ppl, roast in the middle and bottom of the oven, rotating sheets halfway through.)

Meanwhile, cut halloumi into 1/4-inch slices. Rinse halloumi in cold water, then add to a medium bowl. Cover with cold water. Soak for 5 min. Rinse again in cold water, then pat dry with paper towels. Cut tomato into 1/4-inch slices. Stir together pesto and mayo in a small bowl. Set aside.

Heat a large non-stick pan over medium-high heat. When hot, add halloumi to the dry pan. (NOTE: Don't overcrowd the pan; cook in batches for 4 ppl.) Cook until golden-brown, 1-2 min per side.

Meanwhile, halve buns, then arrange on an unlined baking sheet, cut-side up. Toast in the top of the oven until golden-brown, 3-4 min. (TIP: Keep an eye on buns so they don't burn!)

Spread some basil mayo on buns. Stack spring mix, halloumi and tomatoes on bottom buns. Close with top buns. Divide buns and sweet potato wedges between plates. Serve remaining basil mayo alongside for dipping.