In a medium bowl, whisk the balsamic vinegar, Dijon mustard, garlic, kosher salt, and black pepper until the salt dissolves.
Slowly stream in the extra-virgin olive oil while whisking constantly until emulsified and lightly thickened, 1–2 minutes.
Let the vinaigrette stand 10 minutes to mellow, then whisk again and use immediately or transfer to a jar, seal, and refrigerate up to 1 week; shake before serving.