Prepare your preferred version of 90-second bread, making sure to add the caraway seeds before putting it in the microwave. Set aside to cool.
Slice and wash your mushrooms if they aren't already. Melt a teaspoon of butter in a nonstick skillet over medium heat and cook the mushrooms until they're brown and have released their liquid. Put them in a small bowl and set aside.
While the mushrooms are cooking, slice your onion French style (see images above if you don't know what that looks like) and make your burger patty, pressing it as flat as possible. Salt and pepper your patty and put it in the same skillet over medium-high heat. Sprinkle the onions around the burger patty. Cook the burgers for about 6 minutes per side, until cooked through. Stir the onions while the burger is cooking so they don't char.
Slice your bread lengthwise into two even halves. Put a half-slice of cheese on one slice, then top it with a burger patty. Place a whole slice of cheese on the other half and top with the onions and mushrooms. Melt a half tablespoon of butter in the skillet over medium-low heat and grill the sandwich for about 3 minutes per side, turning carefully, until the bread is golden brown and the cheese has melted.
Mix together the mayonnaise, ketchup, and dill pickle relish in a small ramekin. Cut the burger in half on the diagonal to serve and use the dressing as a dipping sauce.
Macros for this recipe are as follows: 864 calories, 7 grams net carbs, 41 grams protein, 77 grams fat.