Gather all ingredients. Heat 1 teaspoon of the oil in a small skillet over medium. Add onion; cook, stirring often, until tender and starting to brown around the edges, 5 to 6 minutes. Transfer to a blender.
Add vinegar, honey, honey mustard, salt, pepper, and remaining ½ cup oil to blender. Secure lid on blender, and remove center piece to allow steam to escape. Place a clean towel over opening; process until smooth and emulsified, about 30 seconds.
Transfer dressing to a clean jar or container with a tight fitting lid. Add poppy seeds to dressing; secure lid, and shake until fully combined. Serve immediately, or store in refrigerator for up to 5 days.
