Melt butter in a large skillet over medium-high heat; add shallots and next 6 ingredients (through cremini mushrooms). Cook 10 minutes or until mushrooms are tender and liquid almost evaporates, stirring frequently. Add vinegar; cook 30 seconds or until liquid almost evaporates.
Heat a large grill pan over medium-high heat. Coat pan with cooking spray or olive oil. Add bread slices to pan. Toast each side of bread.
Assembly: Shave the cheese on top of the bread (since you just toasted your bread, the cheese will melt. (I added the cheese first because I wanted to make sure the mushrooms were visible in the photos and I used a lot of cheese!). Next, add the tomato slices and then add the mushroom mixture. Add a splash of balsamic vinegar and olive oil, season with salt and pepper and add the chives for garnish.