Combine broth, sake, soy sauce, oyster sauce, cornstarch and sugar in a bowl & set aside.
Heat a large nonstick wok or skillet over medium heat; add oil. Once the oil is warm, add garlic and stir-fry until fragrant. Add the broccoli (and any other vegetables if desired) and stir-fry for 3 minutes or so, until slightly tender. Add the sauce mixture and stir constantly until sauce boils and thickens. Serve once sauce and broccoli reach desired thickness and tenderness.