Place the pitted dried plums and pitted dates in a bowl and cover with 1/2 cup hot water; soak for 5–10 minutes while you trim the green beans and slice the mushrooms.
Transfer the soaked plums and dates with their soaking liquid to a blender or food processor. Add the soy sauce, Chinese five-spice, and cayenne. Blend until a thick, smooth brown sauce forms, pausing to scrape down the sides as needed. Transfer the sauce to a small bowl and set aside.
Heat 2 tablespoons avocado oil in a large skillet over high heat until hot.
Add the sliced mushrooms in a single layer and sear without stirring for about 3 minutes. Then stir and cook 1–2 more minutes until the mushrooms are golden brown.
Add the trimmed green beans to the skillet, stir to combine, cover with a lid, reduce heat to medium, and cook for 5–7 minutes, stirring occasionally, until the green beans are bright green and crisp-tender.
Uncover the skillet and pour in the blended plum-date sauce. Stir to coat all the vegetables and cook about 1 minute until the sauce is hot and bubbly. Taste and adjust with salt, pepper, or extra cayenne if desired.
Remove from heat and serve immediately.
