1.Cook lentils in a large saucepan of boiling water for 20 minutes. Add spaghetti and salt; cook until spaghetti is just tender. Drain; return to pan.
2.Meanwhile, heat oil in a large frying pan over medium heat; cook shallot and garlic, stirring, 1 minute or until fragrant. Add mushrooms; cook, stirring occasionally, a further 5 minutes or until mushrooms are tender and browned lightly.
3.Add butter to mushroom mixture; stir over high heat until butter is melted. Stir in pasta mixture and half the parsley; toss until heated through. Season to taste.
4.Sprinkle spaghetti with remaining parsley.