Heat the olive oil in a large frying pan/skillet over a medium-low heat, add the carrot, celery and three-quarters of the diced shallot and saut. For 10 minutes, stirring occasionally, until softened.
Tip the shallot mix into the slow cooker pot and add the lentils, bay leaves and hot stock/broth – the stock/broth should just cover the lentils but not swamp them. Cover, lining the lid with kitchen paper/paper towels to absorb any moisture, and cook for 3½-4½ hours on low or 2½-3½ hours on high, until the lentils are tender and there is minimal amount of liquid in the pot. Leave to cool slightly or to room temperature.
To make the dressing, mix everything together until combined and season with salt and pepper to taste.
Spoon the lentil mixture into a large, shallow serving bowl, add the salad leaves, reserved shallot and dressing and toss until combined. Scatter over the walnuts and cheese, to serve.