the.com/greasy spoon

means An informal, inexpensive restaurant or diner — usually serving fried food — valued for cheap comfort fare rather than ambiance.

from American slang from the early 1900s, built from exactly what it sounds like: cheap diners cooking on well-used grills meant everything — the spoons, the counters, the menus — picked up a permanent sheen of cooking grease. The phrase was originally an insult from food inspectors and snobs alike, but diners and regulars reclaimed it as an affectionate badge of honor for places that prize taste and volume over polish.