Step 1

Preheat oven to 400°F. Trim and discard top 1/2 inch from garlic head. Place the garlic, cut-side up, on a sheet of foil; drizzle with 1 tablespoon oil. Wrap tightly in the foil; roast until the cloves are completely softened and jammy, about 40 minutes. Let cool for 15 minutes. Squeeze the garlic cloves onto a cutting board; discard garlic skins. Using the flat side of a chef's knife, mash the garlic into a paste.

Step 2

Transfer the garlic paste to a lidded jar or medium bowl. Add buttermilk, mustard, honey, lemon zest, lemon juice, onion powder, salt and the remaining 3 tablespoons oil. Seal the jar and shake, or whisk vigorously in the bowl, until well combined. Shake or whisk before serving.