Prep the baguette according to the 3-Hour Baguette recipe while the dough is rising.

Mix brown sugar and balsamic vinegar to create a marinade and coat the steak. Marinate for at least 1 hour or overnight.

Heat grill or skillet over medium-high heat. Cook the steak for 4-6 minutes per side. Let it rest for 5 minutes before slicing.

In the same skillet, heat olive oil and sauté sliced onions with brown sugar for 10-15 minutes. Add mushrooms and cook for another 5 minutes.

Sauté minced garlic in olive oil for 2 minutes. Combine with mayo, sour cream, and freshly chopped herbs until smooth.

Slice the baguette, spread garlic dill sauce, layer steak, caramelized onions, mushrooms, provolone cheese, and arugula.

Toast the sandwich in the skillet over low heat for 3-4 minutes until cheese melts and bread crisps up.