I find pastrami roast to be a lot of fun, it’s a little bit different from the regular cuts I grew up with and it’s so flavorful (I often put one on top of my cholent. It’s unreal!) I bought this in Gourmet Glatt and they have stickers they put on meats educating you as to how best to make them. The sticker on their pastrami roast reads to add balsamic vinegar and brown sugar. I took their advice but added a bunch more ingredients to bring it up yet another notch.
Cook pastrami in its bag according to your butcher’s instructions (about two hours in a 350 degrees Fahrenheit / 180 degrees Celsius oven). Allow to cool, then slice.
To make sauce, heat olive oil in a frying pan over low heat. Add onion and sauté until golden. Add remaining ingredients and let simmer for about five minutes. Pour over sliced roast when ready to serve.