Place the lentils in a medium size saucepan with at least 5 cups of water and 1 teaspoon of the salt. Bring to a boil, lower the heat to medium and continue cooking in a low boil for 30 minutes or until the lentils are tender, but not mushy. While the lentils are cooking, bring a big pot of water to a boil and cook the pasta following the package instructions, reserving about a cup of the cooking water before draining it (remember to generously salt the pasta water). In the meantime, heat the olive oil in a large, deep non-stick skillet with a lid. Add the tomato paste and the remaining 1/2 teaspoon of salt and cook over medium-high heat for 3-5 minutes or until the tomato paste starts to caramelize (it will start to turn a darker red). Add the onion and the garlic, mix well and cook for 10-15 minutes, or until the onion are soft, stirring often. Add the crushed tomatoes and red pepper flakes. Bring to a boil, reduce the heat and simmer, covered, for 20 to 25 minutes. Once the lentils and the pasta are cooked, drain them and add them to the tomato sauce. Add as much of the reserved pasta water as you want (depending on the consistency of the sauce you prefer) and the spinach and toss well. Continue cooking for 3-5 minutes, until the spinach has wilted.
NOTE: Even though you can use regular lentils for this recipe, you will get a better texture if you use small green lentils (also called small French lentils because they stay firmer once cooked.