I agree with you: gastropubs can be annoying. Slightly pretentious food served by a glorified bartender, right? Except when that food is actually delicious.
The burger ($15) at Spur seems like a fancy-pantastrophe* on paper: grass-fed beef, homemade A-1 style sauce, onions cooked down in red wine, a slab of pork belly, and a slice of de-constructed/re-constructed meltable aged cheddar cheese. But when you get it, it all works. For grass-fed beef, the meat is extraordinarily juicy and arrives well-seasoned and on-temperature. It has some of the signature funk of grass-fed beef that plays well with the tart onions. The A-1 sauce is spot on, though the flavor might be heavy for some. I'd ask for it on the side the first time you order it.
* A term I just coined, but will trust you to help pass on.
This might be the burger to convince me that pork belly should come standard. Rather than competing with the burger on a flavor level, it serves much more of a textural role: As you bite into the burger, the pork fat drips down, moistening each bite. Honestly, as I ate it, I totally forgot that there was pork belly on there, instead thinking to myself, "This burger is insanely juicy" (the bun stands up nicely).
The cheese slices are made using the method that British chef Heston Blumenthal developed for his ultra-fancy burger in his BBC4 Perfection series. I've never seen it applied to a commercially available burger before. The concept is to take a very flavorful but not particularly melty cheese (in this case a well-aged cheddar), emulsify it with extra liquid and one or another chemical salt, let it re-solidify, then cut it into slices. When successful, you end up with a cheese that melts like American, but is as flavorful as cheddar. At Spur, they succeed.
Fries come standard with the burger, and are also good—if you're into all crunch. They're cut super thin (second burger I had in Seattle with this kind of fry—is this a thing around Sea-town?), and come with a smoky mayonnaise (little secret: if you ask for it, you can upgrade to a thyme-flavored mayonnaise, which is much nicer).