db={accountants:[{"_id":ObjectId("5e83e96105dc5d88a8f14dcc"),"Name":"abc2","BestClientsIds":[ObjectId("5e83e6d305dc5d88a8f14dc9")],"MyClients":[]}],clients:[{"_id":ObjectId("5e83e6d305dc5d88a8f14dc9"),"Name":"abc","Address":"def","Occupation":"ghi","OtherField":1}]}

db.accountants.aggregate([{"$project":{"_id":1,"Name":1,BestClientsIds:1}},{"$lookup":{"from":"clients","let":{"best_client_ids":"$BestClientsIds"},"pipeline":[{"$match":{"$expr":{"$in":["$_id","$$best_client_ids"]}}},{"$project":{Name:1,Address:1,Occupation:1}}],as:"MyClients"}}])