Heat the olive oil in a large pot over medium heat. Add the diced onion and cook until it softens, about 4 minutes.
Stir in the minced garlic, serrano peppers, and cumin. Cook for 1 minute until fragrant.
Add the chopped tomatillos and cook for 3 minutes, stirring occasionally.
Pour in the vegetable broth and add the rinsed lentils. Bring to a boil, then reduce to a simmer. Cover and cook for 25 minutes until the lentils are tender.
While the stew simmers, grill the corn over medium-high heat for 8 to 10 minutes, turning until lightly charred. Cut the kernels off the cobs.
Stir the grilled corn kernels into the stew along with the salt and black pepper. Simmer uncovered for 5 more minutes.
Ladle into bowls and top each serving with chopped cilantro and a squeeze of lime.