20% fat, or 15% fat maximum. Don't be too rigid, buy on price point and quality. 10% fat will make them dry.
Shop different markets, ask the meat counter where their beef comes from. They should be able to tell you. DO NOT BUY WALMART MEAT.
You live in CA. Dairy farm practices in CA on what they feed the cows would make you cry. Look for out of state beef if you can get it, prefferably ranch raised or midwest. Beware the colorado/wyoming feedlot beef, it might be worse then CA dairy cow beef. YES, what they eat is 100% critical to how it tastes. midwest they get hay, alfalfa and timothy grass, supplemented with corn(silage) ground corn and maybe a bit of soybean meal. Mostly hay, the other stuff costs more
As for preparing,
It is important not to overhandle the patty. You want to scoop up the right amount, smooth it out flat and that's it. If you compress it you will get a poor result, it will be too firm by the time you cook it. There is such a thing as a "tender" burger vs otherwise.
Try to base thickness around how well done you like it. If you like it well done, make it thinner. If you want it to MOO when you take a bite, make it like over an inch thick. Make the burger size(diameter) bigger then the bun you will use by at least 10%.
Alternatively, I learned a trick from "Simply Ming" where he used is thumb to depress the center of his patty to make it thinner in the center. This is a way to make a thicker burger more to the well done side without having to dry it out. Not applicable to rare.
Do not salt until it is being cooked. Some spices like black pepper or garlic can go right away. Anything with salt in it, season salt, etc. do not apply until it hits the grill. For sure not into a "mix" before you make a patty!
On the subject of mix, I am not a fan. However, if you like onion or green pepper with anything hamburger you might consider adding that to the mix. Just remember, kneading it together is also additional handling. I prefer to put onion on after it is cooked, or cook the onions seperate if you like them browned. A kabob stick with layers of raw onion brushed heavily with soy sauce and then grilled until the edges char is perfect to top a burger, IMO.
Seasoning: black pepper to taste, salt(late in the cooking) garlic(powder is better then salt, or mix in fresh if you go that route)
If you want something more fancy, the "montreal steak" season tends to go well depending on what you top with. Old school Lawry's will certainly add flavor, but tends to be a bit too salty. In WI we can get "Mad Dog and Merril" brand of stuff, they are basically famous Packer's tailgaters that have been on their own grilling cookout show for 30 years now.
Gramma Hazel is very good, it is their top seller. I like the up nort pepper as well, but the wife says it is a bit spicy(red pepper flake, but not much in it)
All of their stuff is more spice and less salt then mega brands are.
Do you use ketchup? BOO. The corn syrup and salt in it will ruin your burger. you will note non-fast food places do not put it on theirs, they give it to you if you want to use it.
As for cooking, charcoal is always better then gas. Wood is best. I would pick stove top or a dang George Forman grill over a gas grill, and I had a gas grill for 8 years. Never again.
My personal favorite is a 7/8" thick burger, cooked to not quite medium. I use gramma hazel, and some black pepper. The wife prefers 1/2" thick(pushing well done) with the grill magic(less salt lawry's basically). She loads her burger up with veggies, pickles, a drip of mustard and a dot of BBQ, and a blob of Mayo.
I am particular to melting some blue cheese on them, additional black pepper on top of the blue cheese as it melts, and then either a bourbon BBQ or mild steak sauce(not A1) and either the kabob onions or some onion straws(deep fried battered onion strings, if I have them) and not much else. I do like a loaded up burger too, but usually with just a dap of mustard.
This is WI, there is no such thing as a burger without cheese. I like blue, or 5 year aged cheddar. I have had smoked gouda with a tangy BBQ and an onion ring before. Wife prefers mild cheddar. Do NOT put that kraft singles BS/velveeta on there.
