Steamed Clams with Bacon, Tomato, and Spinach

Cook bacon in a 5- to 6-quart heavy pot over moderate heat, stirring occasionally, until beginning to brown, about 5 minutes. Add onion, garlic, and red pepper flakes and cook, stirring occasionally, until onion is golden, about 6 minutes. Add tomatoes and cook, stirring occasionally, until they begin to break down and sauce thickens slightly, about 8 minutes. Increase heat to moderately high, then add clams and bring to a boil, covered. Cook, covered, stirring occasionally, until clams just open wide, about 6 minutes (discard clams that do not open after 10 minutes). Stir in spinach and cook until wilted, about 1 minute. Season with salt and pepper.