Portabella Mushroom Burger topped with sauteed onion and Swiss cheese

I find myself eating less meat these days.  It is cheaper, for one, and I just feel better incorporating a lot of vegetables into a normal day.  One thing I can't do without is a good cheese burger.  It is something I crave often so I wanted to try a burger with a Portabella mushroom.  It did not disappoint; it does not replace a juicy cheeseburger in my life but it's a great alternative.

Ingredients: (for one burger)

• 1 Portabella Mushroom, wiped with a damp cloth and stem removed
• A quarter of red onion, sliced thin
• 1/4 cup reduced sodium soy sauce
• 1/4 cup vegetable broth
• 1 thick slice of tomato (I used heirloom)
• a handful of arugala
• 2 slices of swiss cheese
• 1 hamburger roll
• olive oil to drizzle

For Honey Mustard:

• 1 tbsp brown mustard
• 1 tsp honey or agave 
• 1 tsp of mayonnaise 

Start by preheating the oven to 400 degrees F.  Place the mushroom on a baking sheet and bake in the oven for about 10 minutes then flip and cook for another 10-15 minutes, until the  mushroom is tender.  Meanwhile, heat a skillet to medium heat and lightly drizzle with olive oil.  Add the onion slices and cook while stirring often for about 10 minutes.  They should have some color, reduce in size and be translucent.  Take the onions out and put aside, in the same skillet pour the soy sauce and stock and heat on medium heat.  Place the mushroom in the liquid as it reduces.  Keep heating until the liquid has evaporated and the mushroom is coated in what has reduced.  Place the onions and cheese on top of the mushroom and cover to allow the cheese to melt.  

To assemble the burger, mix the components of the honey mustard and spread on the bun.  Place all other components on and serve.