In a blender, combine the ingredients for the enchilada sauce. Puree until sauce is smooth. Transfer to a pot, bring to a boil, reduce to a simmer, and let cook until sauce is hot and has thickened slightly, 10 to 15 minutes.
Preheat oven to 375˚ F.
In a medium bowl, combine spinach, lentils, onion, cilantro, lime juice, and salt, set aside.
Warm tortillas by either lightly charring them over your gas stovetop or by dipping them in the warm sauce- they just need to be pliable.
To assemble enchiladas, measure about 1 cup of sauce and spread it evenly in an 8x5 (or 8x8) pan. Next, scoop 1/3-1/2 cup spinach filling into the center of one tortilla, roll gently, and place seem side down in the pan. Repeat with remaining tortillas. Once all tortillas are nestled in the pan, top with 1 so cup of sauce.
Bake enchiladas until lightly browning and bubbly, 20-25 minutes. Remove from oven, let cool slightly, and top with onion, avocado, and cilantro before serving.
Notes
+ Sauce makes enough for two meals. Measure out the sauce you need, then let the remaining sauce cool and freeze in an airtight container.