You could be fooled into thinking Meg’s Hamburgers has been around for decades, with its neon signage, dinerlike pale yellows and light blues, plus the old-school charm of a joint that knows burgers. And Meg’s knows burgers. Here they come as smash patties, adorned simply with special sauce, and cheese if you want it. It’s a purist’s dream on fluffy buns, inside a low-slung brick building in Pioneer Square. Get the beef fat french fries.