In a small bowl, whisk together the milk, lemon juice, and vinegar, and set aside to curdle at room temperature, about 10 minutes.
In a blender, combine the soured milk with the mayonnaise, mustard, and garlic and onion powders. Process on medium-high speed until smooth and well combined, 15 to 20 seconds. Stop the blender and scrape down the sides of the jar with a spatula as needed.
With the blender running on low speed, slowly drizzle in the olive oil until the dressing is emulsified and thickened, about 30 seconds. Add the salt and pepper, adjusting the seasoning to taste.
Transfer the dressing to an airtight container, then stir in the minced herbs and red onion and refrigerate until ready to use. This dressing will keep, refrigerated, for up to 7 days.