Slow cooker vegetable lasagne

Name: Slow cooker vegetable lasagne
Ingredients: 1 tbsp rapeseed oil; 2 onions, sliced; 2 large garlic cloves, chopped; 2 large courgettes, diced (400g); 1 red and 1 yellow pepper, deseeded and roughly sliced; 400g can chopped tomatoes; 2 tbsp tomato purée; 2 tsp vegetable bouillon; 15g fresh basil, chopped plus a few leaves; 1 large aubergine, sliced across length or width for maximum surface area; 6 wholewheat lasagne sheets (105g); 125g vegetarian buffalo mozzarella, chopped