Chicken and cheddar cobbler

Name: Chicken and cheddar cobbler
Ingredients: 75g/2⅔oz unsalted butter; 125g/4½oz pickled pearl onions, drained; 1 large carrot, peeled and finely chopped; 2 sprigs thyme, leaves picked; 2 tbsp plain flour; 500ml/18fl oz chicken stock; 2 tsp Dijon mustard; 150ml/5fl oz single cream; 1 rotisserie chicken, removed from the carcass and torn into bite-sized pieces; 150g/5½oz purple sprouting broccoli, stalks chopped, florets left whole or halved if large; 100g/3½oz baby spinach; salt and freshly ground black pepper; 225g/8oz cold butter, cubed; 475g self-raising flour; 100g/3½oz Irish cheddar, grated