Beef in Oyster Sauce

Name: Beef in Oyster Sauce
Ingredients: 12 ounces beef fillet (tenderloin), pounded and thinly sliced; 1 teaspoon light soy sauce; 1 1/2 tablespoons oyster sauce; Salt and freshly cracked black pepper; Pinch sugar; 2 tablespoons groundnut oil (peanut), divided; 3 cloves garlic, crushed and finely chopped; 1 medium chile, seeded and finely chopped; 7 ounces baby spinach leaves; 1 tablespoon Shaohsing rice wine; Pinch dried chile flakes; 1 teaspoon groundnut oil (peanut); 4 ounces oyster mushrooms; Water, as needed; 1 tablespoon black rice vinegar* (recommended: Chinkiang) or balsamic vinegar; Dash light soy sauce