Make the balsamic dressing: While the burgers cook, halve the buns and place them in the oven to toast for 2-3 minutes, until golden brown. Remove the garlic cloves from the oven and mash with a fork until very smooth. In a small bowl, whisk together 1⁄2 Tablespoon mayonnaise, 1 Tablespoon balsamic vinegar, a large drizzle of olive oil, and as much roasted garlic paste as you like. Season with salt and pepper.