Mix all sauce ingredients together in a small bowl and set aside. In a second small bowl, stir together the cornstarch and water until fully combined. Set both aside near the stove.

Pour 1/4 cup water into a large skillet or wok over medium-high heat. Once boiling, add the broccoli and a pinch of salt. Cover and steam for 1 minute. Uncover and stir until the broccoli is bright green and al dente and the water has fully evaporated. Transfer to a plate.

Wipe the pan clean with paper towels and add 2 tablespoons of peanut oil. Increase to high heat. Add the mushrooms in a single layer and sear for 1 minute without touching. Flip and cook another 2 to 3 minutes until golden brown and all moisture is gone from the pan.

Push mushrooms to one side. Add the remaining 1/2 tablespoon of oil to the empty side, then add the chili peppers, garlic, ginger, and green onions. Stir until chili peppers turn dark red, about 20 to 30 seconds. Stir everything together.

Add the broccoli back to the pan. Pour in the sauce and stir for 30 seconds. Re-stir the slurry to dissolve the cornstarch, then swirl it into the pan. Cook and stir until the sauce thickens and coats everything evenly, about 30 to 45 seconds. Serve immediately.