In a deep saute pan over medium-high heat, cook the bacon until crisp. Remove from the pan and reserve the grease. Add the garlic, lemon zest, and red pepper flakes, and cook until fragrant. The collard greens should be added and cooked until they start to wilt. Cover the pan and reduce the heat to a simmer before adding the chicken stock and 1/2 cup water. Let to simmer for 30 to 40 minutes or until the greens are soft. Remove the lid, re-add the bacon, increase the heat to medium-high, and simmer the liquid for 1 to 2 minutes to reduce it by half. Add the lemon juice and salt and pepper to taste.