Put rinsed lentils and stock in a medium saucepan. Cover and bring to a boil over medium heat. Remove lid and lower heat to simmer and cook stirring occasionally until stock is absorbed. The lentils should be al dente. Drain any remaining broth once they are cooked to this point. Cool.
Combine the onions, peppers, tomatoes, parsley, and string cheese with the cooled lentils in a large bowl.
Put the lemon juice, garlic, balsamic, pepper, and olive oil in a small jar with a tight fitting lid. Shake until well blended.
Pour the dressing over the lentils and vegetables and toss to combine. cover and chill for several hours. Serve