In a 6- to 8-quart pan, combine lentils, broth, celery, carrots, barley, sautéed onions and garlic, and rosemary. Bring to a boil over high heat, stirring frequently. Reduce heat, cover, and simmer, stirring occasionally, 15 minutes.
Add tomatoes, cover, and stir occasionally until lentils are tender to bite, about 15 minutes longer. Let cool to room temperature, about 1 1/2 hours.
Ladle 1/2 the stew into each of 2 plastic freezer bags (1-gal. size). Seal bags and freeze (see Cold Facts below).
Thaw 1 bag (see notes). Empty stew into a 3- to 4-quart pan and bring to a boil over high heat, stirring frequently, about 10 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.