• Gather the ingredients.
  
• Wash the vegetables and pat them dry. Slice the onion and tomato.
• Cut the foot off the mushrooms so the mushroom cap has a level surface. This makes assembling the burger easier and reduces chewiness.

Cook the mushrooms

• Heat the butter in a heavy-bottomed skillet/fry pan on medium heat.
• After the butter melts, add the mushrooms in a single layer.
• Once the mushrooms are brown on the underside, flip them over and cook on the raw side. Monitor to make sure the butter is not smoking or burning in the pan. If your heat is too high, turn it down to the medium-low or even low heat setting.
• Once both sides of the mushrooms are cooked, about 15 minutes, remove them from the heat and set aside.

Caramelize the onions

• Add the sliced onions to the pan with the remaining butter.
• Cook the onion rings in the butter. Add the salt and ground black pepper. Toss periodically to ensure even cooking.
• Once the onions are soft and golden brown, try an onion ring. Don't burn your tongue. If it's caramelized enough to your liking, you can remove the onions from the heat. If they're not ready yet, keep cooking until you're happy with the level of caramelization. Monitor the butter to make sure it's not burning. If it starts to get smoky, add a tablespoon of water.

Toast the buns

• After you remove the onions from the pan, add the buns to the frying pan to toast them. Put them in cut side touching the pan. This step is optional. You can also toast them in a toaster oven if you have one.
• Check the underside of the bun. Once the bun is beginning to turn golden, flip the bun over to toast the exterior side.
• Turn off the heat. Remove the buns from the fry pan if you're worried they might burn. You're ready to assemble the burger.
  

Assemble the burger

• Spread your favorite sauce to the top half of the buns.
• Add the cheese to the bottom half of your buns (the cheese is less likely to fall out if it's on top of the bottom bun instead of the top bun).
• Add the cooked mushroom on top of the cheese.
• Add your remaining toppings. In my burgers, I add a slice of tomato on top of each mushroom, a few caramelized onion rings, and a leaf of lettuce.
• Place the top bun over the lettuce.
• If the burger isn't holding together, you can poke a toothpick through the top bun and push it down to the bottom bun to hold the insides together. Remember to remove the toothpick when you eat the burger!
• Enjoy your mushroom burger!