How to Make It
Preheat oven to 400°F. Cut off and discard tip of garlic head, exposing tops of cloves. Drizzle trimmed garlic head with 1 tablespoon oil, and wrap tightly in aluminum foil. Bake in preheated oven until garlic is tender and lightly caramelized, about 40 minutes. Let cool slightly, about 10 minutes. Gently squeeze garlic cloves from garlic head, discarding skins. Place 1 of the roasted garlic cloves in a food processor. Reserve remaining roasted garlic cloves for another use.
Heat remaining 1 tablespoon oil in a small skillet over medium. Add chopped garlic; cook, stirring often, until garlic is tender but not brown, about 1 minute and 30 seconds. Remove from heat; transfer chopped garlic to food processor.
Add butter, prepared horseradish, fresh horseradish, salt, and lemon zest to food processor; process until combined, about 1 minute, stopping to scrape down sides as needed. Serve at room temperature. Store in an airtight container in refrigerator up to 1 week.