In two separate small bowls, combine the sauce ingredients (vegetable broth, light soy sauce, dark soy sauce, sugar, black pepper, and sesame oil) and mix the cornstarch slurry (cornstarch and water). Set both aside.

Heat a large skillet over medium-high heat and add 1/4 cup water. Once boiling, add the broccoli florets with a pinch of salt. Cover and steam for 1 minute, then remove the lid and continue cooking until the water completely evaporates. Transfer broccoli to a plate and set aside.

Wipe the skillet clean and heat 2 tbsp oil over high heat until shimmering. Add the sliced mushrooms in a single layer. Do not stir for the first full minute to achieve a golden sear. Flip the mushrooms and cook for another 2-3 minutes until browned and all moisture has evaporated.

Push the mushrooms to one side of the skillet. Add the remaining 1/2 tbsp oil to the empty side, then add the chili peppers, minced garlic, minced ginger, and sliced green onion. Stir constantly until fragrant and the chilies darken slightly, about 30 seconds.

Return the broccoli to the pan and mix everything together. Pour in the prepared sauce and stir well to coat all the vegetables. Give the cornstarch slurry a quick stir to recombine, then pour it into the pan. Toss everything continuously for about 1 minute until the sauce thickens and creates a glossy coating on the vegetables. Serve immediately while hot.