Lay a baking sheet and spread walnuts evenly in one layer. Bake for 4-5 min, or until lightly browned. Remove from oven and allow them to cool down completely. Take a large bowl, combine together cooked lentils, onions, parsley, thyme, olive oil, vinegar, salt, and pepper. Put lid and refrigerate for 45 min. While serving, top the salad with toasted walnuts.