I agree with both Peachey and Steve, but there is a bit more to it than just bringing food. These people who bring him cooked food may have lots of reasons for doing it. In most societies the men die before the women, so there are always a lot of widows in the community. In general, these are the people who are the 'casserole brigade'. 1) They think the poor old man who just lost his wife cannot cook for himself (in some marriages, the husband never cooks) 2) They want to see inside his house, because they're nosey and now that grandma is dead they can be critical about her among themselves 3) They want his money 4) These old widows think that he is now a potential new husband for one of them. 5) OK, maybe it's possible that they're just being good neighbours

A brigade is a military unit, a group of soldiers you might say. The author is using a military brigade as a metaphor of a group of people who are coming to his door and bringing casseroles. Hence, casserole brigade. To put it in simple English, many people were bringing casseroles to his house.

The keyword here is "brigade": squadron, force, army... So the casserole brigade is an "army" who makes casseroles! I'm guessing that if Grandma had time on her hands to make casseroles (and other home cooking), she'd have many elderly friends who also made casseroles. There are the people coming to his house... grandma's friends. With casseroles, no doubt. You could add almost any adjective to "brigade"(squad/army) to imply that a hobby (or anything, such as people who have curly hair) is a unified force.