In a large cast-iron or other heavy skillet, heat oil over medium-high heat. Add mushrooms; cook, stirring occasionally, until lightly browned, 4-6 minutes. Add asparagus, garlic, oregano, salt and pepper; cook and stir until asparagus is crisp-tender, 2-4 minutes. Stir in kale; cook and stir until wilted, 2-4 minutes. Remove from heat; stir in fish sauce and vinegar. If desired, top with sesame seeds.