Heat a large high-sided skillet over medium-high heat. When the pan is very hot, add onion and stir constantly for 2 minutes. Add 2 tablespoons water, chile flakes and garlic; cook for 3 minutes, stirring often. When onion is starting to turn translucent, slowly add greens with water still clinging to the leaves, stirring often until wilted, about 5 minutes. Add vinegar, paprika, salt, pepper and 2 cups water. Bring to a boil and then reduce heat to a simmer. Cook uncovered until the pan is nearly dry and collard greens are very tender, 40 to 45 minutes.