In a bowl or measuring cup, whisk together the balsamic vinaigrette, olive oil, shallot, rosemary, Dijon mustard, lemon juice, kosher salt, and black pepper until fully emulsified.
Let sit for about 10 minutes, then taste and adjust salt and pepper as needed.
Notes
Yield: About ¾ cup.
Technique: Whisk until glossy and slightly thickened; shake again if it separates.
Storage: Refrigerate up to 5 days; bring to room temp briefly and shake/whisk before using.