DIAGNOSIS That's polymerized oil—a hard, plastic-like coating formed when oils (especially from lean meats or marinades) heat up and bond to the non-stick surface.
FIX Fill the basket with hot, soapy water and let it soak for at least an hour. Then, use a non-abrasive scrubbing pad (like a blue Scotch-Brite) and baking soda paste to gently scrub. Rinse thoroughly.
IF THAT DIDN'T WORK For stubborn buildup, try a 50/50 mix of white vinegar and water. Soak for 30 minutes, then scrub with the non-abrasive pad. The acidity helps break down the polymerized layer.
HEADS UP Never use metal scouring pads or abrasive cleaners. They will permanently scratch and damage the non-stick coating, making food stick worse and the problem more frequent.