1. 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.
2. Divide the mushroom mixture evenly among four bread slices. Top evenly with Manchego cheese and remaining bread slices.
3. Heat a large grill pan over medium-high heat. Coat pan with cooking spray. Add sandwiches to pan. Place a cast-iron or heavy skillet on top of sandwiches; press gently to flatten. Cook sandwiches 2 minutes on each side or until cheese melts and bread is toasted (leave skillet on sandwiches while they cook). Rub the top and bottom of each sandwich with cut side of garlic clove.