Not much beats In-N-Out when it comes to fast food burgers. Try this out to make a similar burger at home!
Make sauce by combining ketchup, mayo & relish. Set aside.
Dice onion & slice tomato. Prep lettuce.
Flatten meat into patties that are about 1/2 inch larger than the buns. Optionally flavor with salt & pepper.
If you'd like grilled onions. Put in a pan over med-low heat with 1 tsp of oil. Stir often until browned. If oil dries out add 1 tbsp of water to prevent burning. Once done set aside.
Optionally, toast the hamburger buns face down in the onion pan.
Add a tablespoon of oil to cast-iron pan. Place on stove on med-high heat until hot (just beginning to smoke). You may want to turn on a vent fan. (You can grill these as well for a slightly different flavor)
Add meat to pan & cook for about 1.5 min. Add mustard to the top of the burger and then flip.
Place a piece of cheese on top of each patty & finish cooking for about 30 secs to 1 min for medium.
Place sauce on bottom bun, then pickles, tomato & lettuce. Then add a burger, some onions, the second burger and some more onions. Enjoy!