Line a baking sheet with baking parchment. Toss the apple with the lemon juice, agave syrup and cinnamon in a bowl, then spread out on the lined baking sheet and roast in a preheated oven, 160°C (325°F), Gas Mark 3, for 20 minutes. Remove from the oven and leave to cool.
Increase the oven temperature to 180°C (350°F), Gas Mark 4. Pulse all the ingredients for the granola together in a food processor a few times until mixed and mashed. Fold in the cooled roasted apple, then spoon into a lightly oiled 20 cm (8 inch) square shallow cake tin and level with the back of a spoon. Bake in the oven for 20 minutes.
Leave to cool for 15 minutes before cutting into 9 squares to serve.