ROAST CHICKEN THIGHS WITH SALAD ONIONS

2 bunches essential Waitrose Salad Onions, trimmed • 1 garlic bulb, halved • Small handful lemon thyme sprigs • 8 British chicken thighs, trimmed of excess fat • 1 small lemon, zest • 1 tbsp olive oil • 2 x 400g packs Waitrose & Partners Carrot, Swede & Potato Mash • 300g pack fine green beans