Set a large Dutch oven or other deep, wide pot over medium heat. Add olive oil, and when hot, the onions. Saute, stirring often, about 7 minutes, or until onions are translucent and supple. Take care not to let the onions brown – edge heat lower if needed. Add the asparagus , garlic, sea salt, and pepper, and cook just until asparagus starts to soften slightly, about 3 minutes. Add the broth and bring mixture to a boil. Cook about 10 minutes, or just until asparagus is tender and has a little bite. When the cook time has just another minute to go, add the spinach and lemon zest. Remove soup from heat and carefully puree in a blender; puree in batches if needed.