Combine 3 tablespoons sherry vinegar, 3 tablespoons olive oil and minced garlic in a jar with a lid and shake until blended. Set aside.
Place lentils, chicken or vegetable stock, water and 1 tablespoon olive oil in a saucepan and bring to the boil. Reduce the heat to low and simmer, covered, until the lentils are just tender, about 20 minutes. Do not overcook. Remove from heat, drain and cool for at least 30 minutes. Transfer cooled lentils to a mixing bowl.
Stir in the dressing, chives and coriander; mix well. If you're serving the salad immediately, add the goat cheese and tomatoes. If you're preparing the recipe ahead of time, refrigerate the salad and add the goat cheese and tomatoes just before serving.