Garlic Onion Syrup: Combine the onion, garlic, and sugar in a covered bowl and let macerate for 6 hours. Add the mixture and water to a pot and simmer for 15 minutes, stirring occasionally. Cool for 15 minutes, then blend until smooth and strain through a fine mesh strainer. For a fresh salsa-inspired variation, blend in fresh cilantro and jalapeño before straining. Refrigerate for up to 2 weeks.
To prepare a cocktail, shake bourbon, tomato juice, lime juice, garlic onion syrup, and fresh jalapeño with ice. Strain into an ice-filled Collins glass and garnish with cilantro and lime.