[{"body":{"Fields":[{"Name":"description","Value":"Some text"},{"Name":"size","Value":"40"}]}}]

db.collection.aggregate([{$addFields:{body:{$map:{input:"$body.Fields",as:"fi",in:{k:"$$fi.Name",v:"$$fi.Value"}}}}},{"$addFields":{"body":{"$arrayToObject":"$body"}}},{$project:{description:"$body.description",size:"$body.size",_id:0}}])