Roast mejadra onions

Name: Roast mejadra onions
Ingredients: 6 large white onions, peeled; 5 tbsp extra-virgin olive oil; 1 tsp cumin seeds, lightly crushed; 1 tbsp coriander seeds, lightly crushed; 100g white basmati rice; 100g brown lentils; ½ tsp ground turmeric; 1 tsp ground allspice; 1 tsp ground cinnamon; 400ml vegetable stock; 50g flaked almonds, lightly toasted, plus extra to garnish; A large handful of flat-leaf parsley, chopped; 2 tbsp red wine vinegar; A pinch of caster sugar; Rocket salad, to serve (optional)