In a large saucepan or dutch oven, heat butter over medium heat.
Add onions; cook, stirring occasionally, until deep golden brown and softened 30-40 minutes.
Add cornstarch and mix thoroughly; cook 2 minutes longer.
Add the vegetable broth, salt, pepper, garlic powder, and thyme. Bring to a boil.
Reduce heat, cover the pot and let the soup simmer, stirring occasionally , for about 30 minutes.
Turn off the heat, taste, and correct flavors if needed. Let the soup sit in the pot for 2-3 hours to intensify the flavors.