Place lentils, carrot, thyme and salt and pepper in a large saucepan; cover with water. Bring to a boil. Cover pan; cook, simmering, 25 minutes or until lentils are tender. Drain.
Place lentil mixture in a serving bowl with tomatoes and spring onions. Whisk oil, vinegar and mustard and stir in.
Lentils and other legumes are a good source of copper, a trace mineral that may help lower blood cholesterol. The iron content of lentils is more easily absorbed by the body if they are cooked or served with a food that is rich in vitamin C, such as tomatoes or cabbage.