How to Make It
Rinse the lentils and put them into a saucepan with 2 1/2 cups of water and the powdered vegetable stock. Cover and bring to a boil; cook for 20 minutes, until all of the liquid’s absorbed. Put the lentils into a large serving bowl and leave to cool for 20 minutes.
Once the lentils are cooled, finely chop the red onion and put it into a little bowl with the balsamic vinegar and olive oil. Season with salt and pepper; mix and set aside to macerate while preparing the vegetables. Dice the cucumber and cut the cherry tomatoes in half, then add them to the lentils with the black olives.
Cut the halloumi into 8 slices. Heat a griddle pan and grill the halloumi slices on both sides until soft and charred in spots, about 2 minutes. Pour the olive-oil dressing onto the lentils, toss through the arugula and distribute the salad between four plates, topping each with two slices of grilled halloumi.