1. Heat oil in large skillet over high heat. Add onions and 1/4 teaspoon salt; cook, stirring frequently, until the onions begin to brown, about 5 minutes. Reduce the heat to low, add 2 tablespoons water, and cover. Cook, stirring frequently, always covering the pan again, until the onions are reduced and have a deep caramel color, 25-30 minutes.
2. While onions cook, combine 3 cups water and 3/4 teaspoon salt in a soup pot. Add quinoa. Bring to boil. Reduce heat to maintain a simmer, cover, and cook 10 minutes (will look very soupy). Chop asparagus. Add to quinoa, and cook another 5 to 10 minutes, making sure quinoa does not over-cook; may need to add some vegetable broth or water to pan.
3. Coarsely chop the swiss chard and spinach. Add to quinoa and asparagus. Continue to simmer, covered, for 5 minutes.
4. When the onions are caramelized, stir a little of the vegetable broth, making sure to get all the brown bits deglazed. Add the onions to the quinoa. Add vegetable broth and cayenne. Return to a simmer, cover, and cook, stirring once, until the spinach is tender but still bright green, about 5 minutes more.
5. Puree the soup - either in a food processor, blender, or immersion blender in the pot.
Stir in lemon juice. Enjoy!