In a medium saucepan, combine lentils, 2 cloves of garlic, and bay leaf.  Cover with water by 2".  Bring to a boil, reduce heat and simmer until the lentils are tender, 20-25 minutes.  Drain and discard the bay leaf and garlic.

Bring a large pot of water to boil, add the pasta and cook until al dente.  Drain, reserving 1 cup of pasta water.

Slice the remaining garlic cloves.  In a large saute pan, heat the oil and add the garlic, leeks, and thyme.  Cook, stirring occasionally, until the leeks are soft (about 5 minutes).  Add the cooked lentils and season with salt and pepper.  Add the spinach and reserved pasta water to pan.  Toss to wilt the spinach.  Pour the mixture over the pasta, tossing to combine.