In a large 6-quart pot, heat the olive oil over medium-high heat. Saute the onion, carrot, and celery until tender.
Add in the garlic, basil, oregano, and red pepper flakes and stir briefly.
To the vegetables, add in the lentils, maple syrup, crushed tomatoes, water, and 1 teaspoon of salt. (If the tomatoes are unsalted; otherwise start with just half the salt to be on the safe side.) Stir well, then bring the liquid to a boil.
Once boiling, lower the heat to a small simmer and let the sauce cook uncovered until the lentils are tender, about 40 to 45 minutes. Stir every now and then to make sure nothing sticks to the bottom of the pot, and to make sure the liquid isn't evaporating too quickly. (If the heat is up too high, the liquid can evaporate too fast.)
When the lentils are easily smashed with a fork against the side of the pot, they are done. Season the sauce with additional salt to taste. (I usually add another 1 teaspoon, starting just 1/2 teaspoon at a time.) You can also brighten up the sauce with a squeeze of fresh lemon, or a splash of balsamic vinegar, if you prefer.
Serve warm, with your favorite cooked noodles. The leftover sauce can be stored in an airtight container in the fridge for up to a week, or you can freeze it for up to 3 months.
Green lentils give this sauce the best texture because they hold their shape well, but you can make this with red lentils if you prefer. The cooking time will be at least 15 minutes shorter in this case.