Cook the bacon in a deep saute pan on medium-high heat until crisp, then set aside, leaving the fat in the pan. Add the red pepper flakes, garlic and lemon zest, and saute until fragrant. Add the collard greens and cook until they begin to wilt. Add the chicken stock and 1/2 cup water and cover, lowering the heat to a simmer. Let cook until the greens are tender, 30 to 40 minutes. Uncover, add back the bacon, raise the heat to medium-high, and reduce the liquid by one-quarter, 1 to 2 minutes. Add the lemon juice, and season with salt and pepper.