Heat 1/2 cup oil in a large saucepan over medium.
Add red pepper flakes, stirring often, about 4 minutes.
Add collard greens to saucepan a handful at a time, stirring until each addition is wilted before adding the next.
Season with salt and add 1 cup water. Bring to a simmer, then reduce heat so mixture is at a very gentle simmer.
Cover and cook, occasionally removing lid to stir, until greens are tender and very dark green, 1–1 1/2 hours
Let cool slightly, then stir vinegar and sugar into greens.
Transfer to a serving bowl and drizzle with more oil and a splash more of vinegar if desired.
