Preheat the oven to 400°F and move the oven rack to the upper 1/3 of the oven. By moving the oven rack closer to the top element the meat will cook evenly without overcooking the baguette. Line a large baking sheet with parchment.
Slice the baguette through the middle, then cut each portion in half again making 4 pieces. Arrange side by side on the baking sheet. Butter each piece and sprinkle lightly with garlic salt. Cut the cheese slices in half.
Arrange half of the cheese slices over the buttered baguette reserving the other half for the top.
In a skillet over medium-high heat, cook the mushrooms and onions in a couple of drizzles of olive oil until golden.
Add the fresh thyme and season with salt and black pepper to your taste. This will take about 10-12 minutes.
Add a tablespoon of balsamic vinegar and a couple pinches of sugar to sweeten the onions, then cook for an additional 2 minutes. Remove from the heat and set aside to cool slightly.
Mix together all of the steak burger ingredients. Divide evenly between the baguettes, spreading to the edges. The meat will be thin.
Place into the oven and bake for 20-22 minutes depending on the thickness of the meat. Remove from the oven, place the sauteed mushrooms and onions on top, then cover with the remaining cheese.
Set the oven on broil, then return to the oven until the cheese melts.
Cut each steakburger into 4 equal pieces and serve with an assortment of flavored mayonnaise, pesto or spreads.