The optimization process and recommendations:

1. Create Optimal Indexes (modified query below): The recommended indexes are an integral part of this optimization effort and should be created before testing the execution duration of the optimized query.
2. Explicitly ORDER BY After GROUP BY (modified query below): By default, the database sorts all 'GROUP BY col1, col2, ...' queries as if you specified 'ORDER BY col1, col2, ...' in the query as well. If a query includes a GROUP BY clause but you want to avoid the overhead of sorting the result, you can suppress sorting by specifying 'ORDER BY NULL'.
3. Index Function Calls Using Generated Columns (modified query below): When a function is used directly on an indexed column, the database's optimizer won’t be able to use the index to optimize the search. Creating and indexing a generated column (supported in MySQL 5.7) will allow MySQL to optimize the search.

Optimal indexes for this query:

ALTER TABLE `countries` ADD INDEX `countries_idx_country_id` (`country_id`);
ALTER TABLE `sales` ADD INDEX `sales_idx_username_transaction_utc` (`username`,`transaction_utc`);
ALTER TABLE `sales` ADD INDEX `sales_idx_country_id_date_utc` (`country_id`,`date_transaction_utc`);

The optimized query:

SELECT
        lss.`country_id` AS CountryId,
        Date(lss.`transaction_utc`) AS TransactionDate,
        c.`name` AS CountryName,
        lss.`country_id` AS CountryId,
        COALESCE(SUM(lss.`sale_usd`),
        0) AS SaleUSD,
        COALESCE(SUM(lss.`commission_usd`),
        0) AS CommissionUSD 
    FROM
        `sales` lss 
    JOIN
        `countries` c 
            ON lss.`country_id` = c.`country_id` 
    WHERE
        (
            lss.`transaction_utc` BETWEEN '2012-09-26' AND '2012-10-26' 
            AND lss.`username` = 'someuser'
        ) 
    GROUP BY
        lss.`country_id`,
        lss.date_transaction_utc 
    ORDER BY
        NULL