If you like, grate Parmesan cheese over the pasta just before serving, or offer grated cheese on the side.
In a medium saucepan, combine lentils, 1 garlic clove, and the bay leaf. Add enough cold water to cover by 2 inches. Bring to a boil over medium-high heat. Reduce heat to medium-low, and simmer until lentils are tender, 20 to 25 minutes. Drain, and discard garlic clove and bay leaf; set the lentils aside.
Bring a large pot of water to a boil. Add pasta; cook until al dente according to package instructions, about 7 minutes. Drain pasta, reserving 1 cup cooking liquid. Place pasta in a large bowl.
Meanwhile, mince remaining garlic clove. In a large saute pan, heat oil over medium heat. Add minced garlic, leeks, and thyme; cook, stirring occasionally, until leeks are soft but not browned, about 5 minutes. Add cooked lentils and salt; season with pepper.
Add spinach and reserved cooking liquid to pan; toss to wilt spinach, about 2 minutes. Pour mixture over pasta, tossing to combine. Serve immediately.