For the last couple years, I've just been grinding my own and using boneless beef short ribs. That cut has always been my favorite for stews because of its beefy flavor and collagen and fat content, and it works just as well in burgers, yielding a rich, beefy product. Some may say a little too rich, but it's just perfect for me.
I've always preferred thin pan-fried burgers to grilled burgers (heresy, I know), and over the years I've just been using either the
smash technique or pre-forming the patties by putting a ball of meat in between two sheets of wax pepper and slamming down hard with a pan to get it as thin as I can. (Yes, the latter technique results in well-done burgers, or medium-well at best.) Salt-and-pepper only. The meat should be handled as minimally as possible to preserve a light texture. I fry it until it gets nice and crispy brown around the edges. With the first technique, you can actually get something closer to a medium rare if you don't push down on the burger too much and use 1/4 pound of meat or more. Now, pushing down and squeezing out the juices is definitely a no-no on the grill (where it just goes into the fire), but on a pan or griddle, it helps create the brown crust of flavor that is so important to this style of burger. These types of burgers--maybe 1/6 pound and down, 1/4 pounders at the heaviest--are fine well-done. I've never understood people complaining about doneness of thin patty burgers served at places like, say, Five Guys. In my experience, it's pretty much impossible to do these burgers as anything but well done if you want the caramelized edges and the thinness. If I want a medium rare burger, then I go to a 1/2 pound pub-burger style place like Kuma's or Lockdown or whatnot. If I want a fast-food-style burger, which is what I want most of the time when I'm craving a burger, I want the thin patties and crispy edges and that almost always comes with the burger being cooked well-done, or medium-well at best.
Serve with dill pickle slices, mustard, ketchup, and fried or caramelized onions.