THE DINING atmosphere is pleasant, the quiche delicate, the crust flaky. Until ...the lump.

Positive it is neither ham nor mushroom, you summon the manager for an explanation. Unable to determine its origin, he replaces the dish, but when you leave, you're still puzzled -- and slightly queasy.

You may not go to that restaurant again, but someone should know about the experience.

Aside from routine inspections, inspectors at area environmental health departments investigate health-related complaints reported by distressed diners. Dissatisfaction with restaurants runs the gamut from employes not wearing hair restraints, to diners getting sick, to establishments hoarding trash in a back lot. Area sanitation supervisors agree that the most common complaints have to do with unclean facilities, either in the restrooms or in the dining rooms -- where roaches frequently are the culprits.

If you have a beef and call a health department, this is what you can expect: You will be asked to explain the nature of the incident, the location and the date. Health departments do not require that you leave your name, but if you tell them, confidentiality is guaranteed. Even if you remain anonymous, the complaint will be filed. It's just that the department may have a hard time calling you back with its findings after the inspector visits the restaurant.

Some tips for better complaining:

* Call immediately. If you've gotten sick and call the health department a few days later, it's likely that the food you've accused won't be there when the inspector arrives. (All the health departments have labs where potentially contaminated food is analyzed.) And if it's dirt or roaches that you've spotted, the evidence may recur only occasionally.

* Don't jump to conclusions about food-related sickness. Go over the day's eating schedule and other factors that could have added to an upset, before you call. It's hard to prove foodborne illness at a restaurant if you're the only person affected.

* Remember that poor service or disappointing food are not under the jurisdiction of the health departments, but of consumer affairs departments in the county governments.

* Remember to call the health department in the restaurant's jurisdiction.

The following is a list of area environmental health departments.

District of Columbia 724-4113.

Arlington County 558-2661.

Alexandria 838-4855.

Fairfax County 534-0343.

Prince George's County 794-6800 Ext. 217.

Montgomery County 468-4110 for all foodborne illness complaints. All other complaints must be made in writing to: Division of Licensure and Regulatory Services, 100 Maryland Ave., Room 330, Rockville, MD 20850.