Heat oil over medium-high heat. Add onion, cook for 4 minutes. Reduce heat to medium. Stir in mushrooms.Add balsamic vinegar and stir. Cook for 5 minutes, stirring occasionally.
Add spinach and tomatoes and cook for another minute. Stir in mustard and turn off heat. Season with salt and pepper. Drain off excess liquid and reserve. Place spinach mixture in the refrigerator and cool for 15 minutes.
Form bison into 4 equally sized balls. With your fingers, make a well in the center of each ball. Stuff about 2 tablespoons of spinach mixture inside each ball. Seal the top and flatten to form a patty.
To grill: Heat a grill pan using a little oil. When hot, grill each patty for about 2-3 minutes on each side or when desired oneness is reached. Repeat, as needed, depending on pan size.
To broil: Set oven broiler to “high.” Place patties on a foil–lined baking sheet on top oven rack. Broil for about 3 minutes, flip patties over, and broil for about another 2 to 3 minutes or until burgers reach desired denseness.
Let the burgers rest for a couple of minutes to serve, take 2 romaine leaves and sandwich burger between them