Bring a large pot of salted water to a boil over high heat. Add the lentils and cook until tender, stirring occasionally, about 18 to 20 minutes.
Drain and let cool for 5 minutes. (If you're using packaged, cooked lentils you'll only need to rinse and drain them.) Place lentils and remaining salad ingredients in a large salad bowl.
Place the lemon juice in a small bowl. Slowly add the oil, whisking constantly, until combined. Season with salt and pepper, to taste
Pour the vinaigrette over the salad and toss well.