Preheat oven to 180°C. Thinly slice onion. Heat a drizzle of oil in a medium pot on medium-high heat and cook onion for 5-7 minutes, stirring occasionally, until soft. Add sugar, a drizzle of balsamic vinegar and a splash of water and stir to combine. Cook for a further 1 minute. Remove from heat and set aside.
Peel cucumber into ribbons and chop mint leaves. Add cucumber and mint to a bowl with spinach and a drizzle of vinegar and oil and toss to combine. Season.
Slice burger buns in half and place cut side up on a lined oven tray. Warm in oven for about 5 minutes. Drain capsicum and slice 2cm.
Pat haloumi dry, slice into 1cm pieces and sprinkle over spices. Heat a drizzle of oil in a medium fry-pan on medium heat and cook haloumi for 1-2 minutes each side, until golden.
Serve buns filled with haloumi, onions, capsicum, relish, pesto and salad. Serve any remaining salad on the side.