Peel, halve, and thinly slice 3 onions. In a large skillet with lid over high heat, warm 1 TB olive oil. Sauté onions 5 minutes. Sprinkle with salt, reduce heat to medium, and sweat those onions for 15-20 minutes, stirring every few minutes to keep onions from sticking and allow them to brown evenly. Meanwhile, remove ribs from, and chop 1 1/2 lbs. kale. In a small frying pan over medium heat, toast 1 TB fennel seeds until fragrant, about 1 minute. Remove fennel seeds. Toast 1/4 cup shelled sunflower seeds (optional). Add kale, 1/2 cup vegetable broth and fennel seeds to the skillet. Stir in kale. Cover and cook on medium-low for 7-8 minutes, then uncover and continue to cook, slowly until greens are very tender, another 8-12 minutes (depends on temperature and the robust quality of the kale). Stir in 1/2 TB balsamic vinegar. Salt and pepper to taste. Sprinkle with toasted sunflower seeds, if using.