Ihre Methode sollte wie folgt aussehen:
public async void Insert()
{
var client = new MongoClient("mongodb://localhost:27017");
var database = client.GetDatabase("foo");
var collection = database.GetCollection<BsonDocument>("bar");
var document = new BsonDocument { { "_id", 1 }, { "x", 2 } };
await collection.InsertOneAsync(document);
}