I don't think the body.content field is supported as filter item. In your case I think you may be better off with a search.
Try Search with filters and see if it gives what you are looking for.
POST https://graph.microsoft.com/v1.0/search/query
{
"requests": [
{
"entityTypes": [
"message"
],
"query": {
"queryString": "subject:'Lunch' AND somestring-that-will-match-content"
}
}
]
}