"Gailan is a great leafy green vegetable that makes for a great stir-fry. Made this for dinner and couldn't wait to have the leftovers the next day for lunch. Serve with rice or tossed with your favorite noodle. I'm not a vegetarian or anything, but like the value and ease of preparation of tofu in place of chicken."
Place tofu slices between layers of paper towels. Place a baking sheet over the tofu. Set a 3- to 5-pound weight on top (a container filled with water works well). Press tofu for 20 to 30 minutes; drain and discard the accumulated liquid.
Combine oyster sauce, soy sauce, sugar salt, and red pepper flakes in a bowl. Whisk sauce until sugar is completely dissolved.
Whisk 2 tablespoons water with cornstarch in a separate bowl to make a slurry.
Heat peanut oil in a large wok over medium-high heat. Cook and stir ginger and garlic until fragrant, about 2 minutes. Add gailan; toss to evenly distribute ginger and garlic. Add water and cover; steam for about 2 minutes. Stir in the sauce, tossing quickly to coat. Reduce heat to medium; cook for 1 minute.
Stir the slurry briefly; add to the gailan. Cook and stir until sauce is thickened, 2 to 3 minutes. Garnish with sesame oil.