In a large dutch oven or pot, cook the bacon over medium heat until all the fat is rendered. About 5 minutes before the bacon is fully cooked, add in the onion and cook until translucent over medium/low heat. Once the onions are cooked, add the garlic and toast until fragrant. Add the apple cider vinegar, red pepper flakes, and chicken stock. Add collards and stir. Bring to a boil. Once at a boil, turn down to simmer and cover. Cook for 1 hour (or longer) stirring occasionally until the collards are tender. Once the collards are tender, season with salt and pepper to taste.