Toast the English muffin halves until golden.
While still warm, spread 1 teaspoon (5 ml / 0.2 fl oz) of butter on the cut side of each half and set aside.
Prepare your vegetables. Gently wipe the portobello mushroom clean with a damp cloth and cut it into thin, even slices. Wash and pat dry the romaine lettuce leaf.
In a skillet over medium heat, warm the olive oil.
Add the chopped onion and cook, stirring occasionally, until tender and fragrant.
Add the remaining 1 teaspoon (5 ml / 0.2 fl oz) of butter to the skillet. Push the cooked onions to one side and arrange the portobello mushroom slices in a single layer.
Cook for about 2 minutes until the bottom is browned.
Flip the mushroom slices and cook the other side until browned.
While the second side is cooking, add the tomato slice to a space in the pan to warm it through.
Once the mushroom slices are browned on both sides, sprinkle the grated Swiss cheese evenly over the top.
Continue to cook for another minute, or until the cheese is perfectly melted and bubbly.
Assemble your burger.
Place the warmed tomato slice on one muffin half and the sautéed onions on the other. Carefully lay the cheesy mushroom slices over the top and garnish with the fresh lettuce.
Serve immediately, open-faced.
Don't have Swiss cheese? Provolone, Gruyère, or even a sharp cheddar melt beautifully and pair wonderfully with the earthy mushroom.
