Add olive oil to a small sauce pan over medium-high heat. Once the oil is shimmering, add garlic and cook it for 1 minute.
After a minute, add and stir in hoisin sauce, rice vinegar, sherry, soy sauce, ketchup, oyster sauce and water. Stir everything to combine well.
Simmer the sauce for 20 minutes stirring occasionally.
After 20 minutes, the sauce should have thickened up. Add sesame oil, stir well and remove the sauce from the heat.
Allow the sauce to cool and store in the fridge for up to two weeks.