Soften the butter with the oil in a 12-inch skillet over medium warmth. Utilizing a big spatula, switch the patties to the skillet, onion facet down. Prepare dinner for 6-8 minutes, till the onions flip a deep golden brown and start to crisp across the edges. Flip the burgers, then improve the warmth to excessive and prepare dinner till properly browned on the second facet, about 2 minutes.
Add 1 or 2 slices of cheese to every burger and permit to soften. Add mayo, mustard and pickles to the highest buns, then place every burger on a backside buns. Serve instantly.