• Combine 1/2 cup balsamic vinegar, brown sugar, garlic, and lots of salt and pepper into a large Ziplock bag then squish to combine. Add steak then marinate for 30 minutes or up to 8 hours.
• Heat extra virgin olive oil in a large skillet over medium heat. Add onions, season lightly with salt, then saute until caramelized and soft, 30 minutes, stirring occasionally and turning heat down slightly if onions begin to sear or burn. Stir in remaining 1 Tablespoon balsamic vinegar then set onions aside to cool - can be made 2 days ahead of time.
• Preheat a grill over high heat for at least 10 minutes then turn heat down to medium-high. Add steak then grill for 3-4 minutes per side, depending on how thick your steak is, then let rest for 10 minutes before thinly slicing against the grain.
• Divide lettuce, steak, caramelized onions, peaches, and cheese crumbles between bowls then top with vinaigrette and serve.

For the Honey-Balsamic Vinaigrette:

• Add ingredients into a blender then blend until smooth. Can be made 3-4 days ahead of time.