Heat a large, heavy-bottomed pot over medium-high heat. Add the bacon slices and cook for two to three minutes on each side, until brown. Remove two of the 2-inch strips and set aside (leave the rest in the pot). Add the onion and garlic and cook for five to six minutes, stirring occasionally, until very fragrant. Add the spinach and cook, stirring continuously, for four to five minutes, until it begins to wilt.