L.S.
pour baking soda in and let sit over night, alot of baking soda, like the whole small box, one box on each side. in the morning follow with lots of vinegar to make the baking soda bubble and froth, buy a gallon of vinegar. follow with very hot water, like boiling water. i would do both sides of the sink. then i would also add the septic powder of microbes to get anything else left in the pipes, and let that sit overnight, the next night or for several nights in a row, then repeat baking soda/vingera. the baking soda and vinegar usually works at our house. you can also try lemon juice, and let that sit in the trap all night. but you might need to get a big bottle of it.
might have the plumbers check the lateral line or septic for you, or just call them for advice. it may something simple to fix or something that they can advise you to have checked out.
good luck,
L.