In a medium bowl add add balsamic vinegar, dijon, olive oil and garlic and whisk together until combined. Remove stems from mushrooms and line on a plate. Brush balsamic marinade on both side of mushrooms . Allow to marinade for at least 15 minutes.
Heat a bbq or indoor grill pan to medium-high heat. Grill mushrooms for approximately 5 minutes per side, melting havarti cheese over the mushrooms for the last minute or two of cooking. Optional - toast slider buns the last two minutes on the grill.
Arrange sliders with lettuce, portabella mushroom and tomato slices on the bun. Serve immediately.
Recipe by Good Thyme Kitchen at https://www.goodthymekitchen.com/portabella-mushroom-sliders/