Heat 2 T oil in a large skillet over high heat. Add onions and 1/4 t salt; cook, stirring frequently, until the onions begin to brown, about 5 minutes. Reduce the heat to low, add 2 T water and cover. Cook, stirring frequently until the pan cools down, and then occasionally, always covering the pan again, until the onions are greatly reduced and have a deep caramel color, 25-35 minutes.
MEANWHILE, rinse lentils and pick out any small stones; combine the lentils with the remaining 4 cups water in a soup pot or dutch oven. Bring to a boil. Reduce heat to maintain a simmer, cover and cook for 20 minutes. Trim the white ribs out of the chard; chop the greens and slice the ribs (keep in separate piles). Cut potato into 1/2-inch dice. Chop spinach; set aside.
When the lentils have cooked for 20 minutes, stir in the chard ribs, potato, scallions, broth and the remaining 1t salt; return to a gentle simmer. Cover and cook for 15 minutes.
Stir in the chard leaves, broccoli, cumin and coriander. When the onions are caramelized, stir a little of the simmering liquid into them, add them to the soup. Return to a simmer, cover and cook 5 minutes more. Stir in the reserved spinach, cilantro, mint, jalapeno ad pepper; return to a simmer, cover and cook until the spinach is tender but still bright green, about 5 minutes more. Stir in 1T lemon juice. Taste and add more lemon juice and/or pepper, if desired.
Garnish each bowl of soup with crumbled feta cheese.