Fill bowl with cold water and add teaspoon of salt. Cut eggplant into lengths about 2 inches long and a half-inch thick. Immediately submerge pieces in salted water (will prevent eggplant from browning). Let soak for 15 minutes, then drain eggplant well. Set aside.
In a bowl, mix the cornstarch coating ingredients. Add eggplant slices and mix with your hands so eggplant pieces are completely coated.
Mix the cornstarch slurry and set aside.
Fill heavy bottomed pot 2-3 inches deep with oil and heat. Use a thermometer to ensure oil reaches 365°F/185°C. Fry eggplant in small batches (to maintain high temperature) and turn pieces after a minute so they cook evenly.
Fry each batch for about 3 minutes or until the exterior is just turning a little golden. Drain eggplant on a paper towel-lined rack.
Place green beans into a large skillet and cover with water. Bring to a boil, then reduce heat to medium-low and simmer until beans start to soften, about 5 minutes. Drain water. Add butter to green beans. Cook and stir until butter is melted, about 2-3 minutes. Set aside.
After all eggplant is fried, heat a tablespoon of oil in a large pan or wok over medium-high heat. When pan is hot, add black bean sauce and minced garlic. Stir-fry for 1 minute (don't let garlic burn).
Add scallions and red chili to pan. Stir-fry until scallions get bright, about 30 seconds.
Add soy sauce and mushroom sauce to pan. Give the cornstarch slurry a mix and add it to pan as well, stirring quickly. Turn off heat as sauce thickens.
Add fried eggplant and fold gently with a spatula to distribute the sauce. Mix with cooked green beans. Garnish with chopped scallions.