Chop fresh rhubarb into small pieces, about 1-inch long. Place in a saucepan with orange juice and a splash of water. Bring to a boil, then simmer for about 5 minutes until tender.

Remove from heat and let cool for a few minutes. Transfer to a food processor and blend until smooth.

Add grated onion, garlic, rhubarb vinegar, honey, salt, and pepper to the processor. Blend until well incorporated.

With processor on low, slowly drizzle in the olive oil, blending for about 30 seconds until smooth and creamy.

Pour the vinaigrette into a clean jar and store in the refrigerator for fresh use.