Fast food chains like Burger King and McDonald’s aren’t known for being particularly healthy, gourmet, or sustainable. It doesn’t matter if tomatoes or potatoes aren’t in season–McDonald’s customers want their fully-garnished burgers and fries. And factory-farm meat, well, that’s always in season.

Enter Burgerville, a 39-restaurant gourmet fast food chain in the Pacific Northwest. The chain already uses local and sustainable ingredients in its food, and now Burgerville is taking its commitment to the locavore movement a step further with new seasonal food combinations. Each menu item highlights a single in-season ingredient sourced from local farms committed to sustainable practices.

This month, for example, Burgerville is featuring a rosemary chicken sandwich and rosemary shoestring potatoes, priced at $5.99 and $2.99, respectively. April’s spinach focus will bring a spinach florentine pastry and spinach salad. Many of Burgerville’s items come from family farms like T. Malatesta Farms in Canby, Oregon and Liepold Farms in Boring, Oregon. In the past, such small farms have been virtually ignored by the fast food industry.

Meat eaters at Burgerville can also rest easy knowing that they are eating antibiotic and hormone-free beef. Burgerville’s supplier is Country Natural Beef, a company whose sustainable practices are third-party certified by the Food Alliance.

So far, Burgerville’s strategy is working. Company CEO Jeff Harvey believes that Burgerville’s continued growth and expansion is largely due to its commitment to sustainability. Just today, the company was recognized by Greenopia with a three out of four leaf rating for its sustainable practices.

Burgerville isn’t the only green fast food joint to pop up recently. Seller’s Markets in San Francisco, O’Burger in LA, and Nat. in Berlin, Germany all feature free-range meat and organic food options. But none of these restaurants have expanded to as many locations as Burgerville, and none of them feature rotating menus. Local, seasonal food is often presumed to be the exclusive domain of the rich–or at least those who have access to the new farmers market-locating Locavore iPhone app. Perhaps Burgerville can begin to change that.