In a large pot, heat oil over medium heat. Add onion, garlic, crushed red pepper and cilantro, along with a large pinch of salt, and cook, stirring, until onions are softened and just barely beginning to brown, about 10 minutes.
Add lentils and broth to pot, plus 2 1/2 cups water. Raise heat to high and bring mixture to a boil. Lower heat and simmer until lentils are just tender, about 30 minutes longer.
Add yellow squash and Swiss chard and cook, stirring occasionally, until vegetables are softened but still bright in color, about 5 minutes. If you would prefer a thinner soup, add more broth or water 1/2 cup at a time. When soup is the consistency you like, stir in lemon juice and season to taste with salt and pepper. Ladle soup into bowls and serve immediately.