I saw this on youtube a few days back, thought I would try and duplicate it.  I did not go back and check it again, tried to go from memory, and that is why it did not come out as good as I believe it could.  Will make a few adjustments on next one...size of meatball (way to big), amount and thinness of onions and how the bun is placed on cooking burger towards end.  Need much fresher buns also!

That looks mighty tasty!  You could call it a hamburger steak on a bun.

I know when I do that kind of thing I slice the onions very finely on a mandolin and get the blackstone nice and hot, the onions go down and then you smash the meatball into the onions. I usually do 2 ounce patties for smash burgers and do a pair of patties per burger. I find it gets that wonderful crust that way and cooks super quick.

I did 2 last night with 3oz balls and fresh brioche bun and it was a much better product!  I like the onions on top, as I do not have to deal with the meat sticking when I smash!

 I did burgers last night. I used 1/3 lb. per burger, pressed a dimple in the middle and fried in a large 400° cast iron skillet with a smaller cast iron skillet on top of three burgers for ~4 minutes/side. The sear was perfect. I'll spring for a better weight soon. My smaller skillet weighs 5 lb. 3 oz. What is the recommended weight for a press?

I did burgers last night. I used 1/3 lb. per burger, pressed a dimple in the middle and fried in a large 400° cast iron skillet with a smaller cast iron skillet on top of three burgers for ~4 minutes/side. The sear was perfect. I'll spring for a better weight soon. My smaller skillet weighs 5 lb. 3 oz. What is the recommended weight for a press?

I doubt you  will find  one any where near that heavy. Would probably be something you would have made out of 3/8 or 1/2' steel depending on the diameter you are looking for.

For a reference point, this is 2 lbs.  It was really made for bacon, but works well for steaks.  it is what i use to smash burgers, but I do not keep weight on them.

I do not keep weight on them.

IMHO, keeping the weight on plus high temp.is what yielded the great sear.

I am sure it does.  A crust on a burger has never mattered to me.  I am just a fan of a thin burger!