In a large deep pan, saute the chopped onion in 2 tablespoons of vegetable oil until soft but not brown, about 10-12 minutes. Add the flour and mix it in with onions, cook for 4 minutes. Slowly add the 2 cups of stock stirring constantly to avoid lumps. Add the sugar, lemon juice, vinegar, and raisins, season with 1 teaspoon salt. Taste and add more salt and/or sugar as needed. Thicken with cornstarch slurry to desired consistency.