Add sausage and sauté until heated through. Stir in garlic and sauté for about 30 seconds to 1 minute – do not brown.
Add chicken stock and black-eyed peas to the pot. Bring to a boil; cover and reduce heat to simmer. Allow to cook on low until peas are cooked, about 2 to 3 hours.
Stir in collard greens and continue cooking until greens are wilted, approximately 5-10 minutes.
Add vinegar, red pepper flakes, thyme and salt and pepper to taste.