Herb and Goat Cheese Dutch Baby with Jammy Balsamic Onions

Ingredients

• 2 Tbsp. unsalted butter
• 4 large eggs
• 1 1/2 cups milk (see notes)
• 1 1/2 cups all-purpose flour
• 1 tsp. kosher salt
• 1/2 tsp. ground black pepper
• 1 cup, packed, finely chopped fresh soft herbs (see notes)
• 4 oz. goat cheese, crumbled
• 1 Tbsp. extra-virgin olive oil
• 2 medium yellow onions, thinly sliced
• 1/4 cup balsamic vinegar
• 1 Tbsp. brown sugar