In a large, heavy-bottomed pot over medium heat, combine onions, red pepper, 1/2 cup of the broth, jalapeño, garlic, salt, and pepper. Cook, stirring frequently until liquid is evaporated and onions are translucent (about 10 minutes).
Add remaining 3 1/2 cups broth, squash, lentils, lime juice, chili powder, cumin, smoked paprika, and oregano. Bring to a boil.
Reduce heat, cover, and simmer for 15 minutes, stirring occasionally. Remove lid and cook, stirring frequently, for an additional 15 to 20 minutes, until lentils are thoroughly cooked and squash is tender but still holds its shape. If mixture seems too thick, add a little water as needed. Chili is done when most of the liquid has been absorbed.
Stir in 1/4 cup of the cilantro, if using. Taste for seasoning and add more lime juice and/or spices as needed.
Divide into 6 bowls. Garnish each bowl with a sprinkle of the remaining cilantro, if desired. Serve each bowl with a warmed whole wheat or corn tortilla, if desired.