Heat oil in a large pot over medium heat. Add garlic; gently sauté until fragrant and golden brown, about 1 minute. Pour in chicken stock and add turkey leg; cover and simmer for 30 minutes.

Add collard greens and increase heat to medium-high. Cover and cook, stirring occasionally, until greens have cooked down, about 45 minutes.

Reduce heat to medium. Season with salt and pepper to taste; cover and continue to cook until greens are tender and dark green, 45 to 60 minutes.

Drain greens, reserving cooking liquid. Mix in red pepper flakes if desired. Use reserved cooking liquid to reheat leftovers.