Preheat oven to 400 degrees F (200 C).

Remove tough ends of the Brussels sprouts and halve them. Try to keep them evenly sized; quarter very large ones if necessary.

Place Brussels sprouts and onion onto a baking sheet. Add olive oil, salt, and pepper. Toss to mix well. Arrange the Brussels sprouts so the flat sides face down. Arrange larger pieces on the edge of the baking sheet because they’ll cook faster.

Bake on the middle rack for 15 minutes. Flip and bake for another 5 to 10 minutes, until the sprouts turn crispy and cooked through.

(Skip this step if using pre-made homemade Kung Pao Sauce) While baking the Brussels sprouts, prep and cook the Kung Pao sauce. Mix Chinkiang vinegar, soy sauce, sugar, cornstarch and salt in a small bowl. Add oil and Sichuan peppercorns in a big pan, and cook until hot. When the peppercorns turn dark brown, scoop out and discard. Add dried red chili pepper. Stir until the peppers turn dark brown. Add minced garlic and ginger. Stir a few times. Stir the vinegar based sauce to dissolve the cornstarch completely; add to the pan. Cook until the sauce thickens and remove from the stove immediately.

Once the Brussel sprouts are cooked, add the roasted cashew and Kung Pao sauce (or pre-made Kung Pao Sauce). Gently toss a few times and serve immediately as an appetizer, or as a main dish over steamed rice.