Can I use $project to return a field as the top level document in a mongo aggregation query?

Yes, you can use $project to do that. You just have to tell it to retrieve the nested contents object using dot notation:

db.items.aggregate( {$project: {contents:'$field1.contents'}} );

Additionally, if you want to to hide the _id field from the output, you can specify _id: 0 in the $project parameters:

db.items.aggregate( {$project: {contents:'$field1.contents', _id:0}} );

• Starting Mongo 3.4, the $replaceRoot aggregation operator can be used to replace a document by another (in our case by a sub-document):

  // { field1: { content: { a: 1, b: 2 } }, field2: { othercontent: {} } }
  // { field1: { content: { c: 1, d: 2 } }, field2: { othercontent: {} } }
  db.collection.aggregate({ $replaceRoot: { newRoot: "$field1" } })
  // { content: { a: 1, b: 2 } }
  // { content: { c: 1, d: 2 } }
  

• Starting Mongo 4.2, the $replaceWith operator can also be used:

  db.collection.aggregate({ $replaceWith: "$field1" })