Rinse and drain chard. Trim discolored ends from stems. Thinly slice stems and coarsely chop leaves.
In a 5- to 6-quart pan over medium-high heat, stir pancetta often until lightly browned, about 5 minutes.
Stir in chard, sautéed onions and garlic, broth, lemon juice, tomato paste, and thyme. Bring to a boil over high heat, stirring frequently. Reduce heat, cover, and simmer until chard stems are tender-crisp to bite, 2 to 4 minutes; stir occasionally.
Stir in beans. Let cool to room temperature, about 1 1/2 hours.
Ladle 1/2 the soup into each of 2 plastic freezer bags (1-gal. size). Seal bags and freeze (see Cold Facts below).
Thaw 1 bag (see Thawing below). Empty soup into a 3- to 4-quart pan and bring to a boil over high heat, stirring frequently, about 15 minutes. Season to taste with salt and pepper.
Thawing: Cooking times are for thawed, chilled foods. Overnight: The night before serving, place container of frozen food in the refrigerator. Quick-thaw: Heat frozen foods in plastic bags in a microwave oven at 50% power just until pliable, then cook as recipe directs or empty food into a microwave-safe container, drape loosely with microwave-safe plastic wrap, and cook at full power (100%) until steaming. Note: If your microwave accepts metal, you can also quick-thaw dishes frozen in foil pans; follow manufacturer's instructions.
Cold Facts. Packaging: freezer bags. If ingredients include liquid, set each plastic freezer bag upright in a bowl, then fill. Squeeze out all air and seal bag. For better protection of frozen foods during storage, seal each bag inside a second one. Lay bags flat in freezer until solid so they will stack.
Note: Nutritional analysis is per serving.