Light a grill over medium heat or heat a skillet over medium-high heat with a drizzle of olive oil. Divide the ground beef into four equal portions. Roll each section into a round shape.
Place the ground beef rounds on the grill or in the skillet and press down slightly on each, flattening them. Cook for approximately 5 minutes per side, or until the internal temp reaches 160 degrees F.
While the burgers are cooking, melt the butter in a small skillet over medium heat. Add the onion slices and mushrooms and cook for 5 minutes, or until fragrant and golden brown. Transfer the veggies to a bowl or work surface. Do not clean out the skillet.
Place the cut sides of the buns face down in the hot skillet for 1 minute. Transfer to work surface.
Assemble each burger: Bottom bun with 1 tbsp each of ketchup and mustard spread on it, 1/4 of the mushroom-onion mixture, burger, 1 lettuce leaf, 1 tomato slice, 4 pickle slices, top bun with 1 tbsp mayo spread on it.