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To make the onion sauce, heat a small saucepan over medium-high heat with the olive oil. Add onion, garlic, salt and pepper, and cook for 5-7 minutes until caramelized and brown. Add tomato paste, stir and cook until fragrant, about 30 seconds. Deglaze with beer, add vinegar and brown sugar and bring to a bubble.
In a separate bowl, whisk together cornstarch and 1 tablespoon of water. Stir the slurry into the bubbling beer/onion sauce. Once thickened, about 1-2 minutes, remove from heat.