Cook the onions in a pan with the butter until soft, add the lentils to the pan and cover with the chicken stock and bring to the boil and simmer for 15-20 minutes until just tender. Drain, and then toss with the lemon juice, garlic and 2tbsp of olive oil and season. Cool and place in fridge. Add the tomatoes and pistachio nuts to the lentils and transfer to a serving platter. Using a mandolin, thinly slice the cucumber lengthways, and arrange on top.