Boil the lentils in a pot of water until they are done, yet still crisp, approximately 20-30 minutes. Do not overcook or they will be mushy. (Check frequently.)
Drain and rinse lentils, and set aside.
Heat oven to 450F degrees.
Place the sweet potato slices in a bowl and top with about 2 tablespoons olive oil.
While potatoes are cooking, add a little water to the apple pieces and cook them in the microwave for about 3 minutes. Drain.
Whisk together remaiing olive oil, lemon, salt, pepper and garlic until emulsified.
In a large bowl combine the lentils, sweet potatoes, apples and onions mixture.
Pour the dressing over the salad and mix gently.