— 1/4 cup chopped onion
— 600 grams of ground beef
— 1 tablespoon Worcestershire sauce
— olive oil
— 4 hamburger buns
— 4 portabello mushrooms, sliced thin
— 4 slices of Swiss cheese
— 3 tablespoons balsamic vinegar
— 1/2 cup mayonnaise
— 1 cup spinach
— 2 tomatoes in slices
— chopped chives
Preheat grill on medium-high heat.
In a skillet, sauté onion over medium heat for 3-5 minutes until soft.
Mix onion with ground beef, Worcestershire sauce, salt, and pepper.
Form 4 patties from the ground beef.
Put a few drops of olive oil on top of the hamburger meat.
Toast the hamburger bun.
Grill hamburger meat for 4-5 minutes on each side (for medium rare).
Heat the skillet again over medium heat and add a little oil and the sliced mushrooms.
Season the mushrooms with salt and pepper and sauté until soft, about 3-5 minutes. Get out of the fire.
In a small bowl, mix balsamic vinegar, mayonnaise, and freshly ground pepper.
Prepare the burgers: put the meat on the bun, then the Swiss cheese and on top the mushrooms.
Spread the tops of the bread with the balsamic mayonnaise and place spinach leaves.
Serve with sliced tomatoes with olive oil, salt, pepper and chopped chives on top.