Preheat oven to 425 degrees

Thoroughly rinse produce and pat dry

Prepare a baking sheet with foil and cooking spray

Trim ¼" off ends of Chinese broccoli and halve stalks, separating stems from leafy tops. Cut stems into 2" pieces. Coarsely chop tops. Mince garlic. Trim and cut green onions into ½" pieces. Coarsely chop peanuts. Season steak strips with a pinch of salt and pepper.

Bring a small pot with rice and 1½ cups water to a boil. Reduce to a simmer, cover, and cook until tender, 18-20 minutes. Remove from burner and fluff rice. Cover again and set aside. While rice cooks, roast broccoli.

Place Chinese broccoli, stems and tops, on prepared baking sheet. Toss with 2 tsp. olive oil, ¼ tsp. salt, and a pinch of pepper. Spread into a single layer and roast until tender, 6-8 minutes. While broccoli roasts, start steak.

Heat 2 tsp. olive oil in a medium non-stick pan over medium-high heat. Place sirloin steak strips in hot pan. Cook undisturbed until browned, 1-2 minutes. Transfer to a plate. Steak strips will finish cooking in a later step. Reserve pan; no need to wipe clean.

Add garlic and 1 tsp. olive oil to pan used to sear steak strips over medium-high heat. Stir constantly until fragrant, 30 seconds. Add soy sauce, brown sugar, and a pinch of red pepper flakes (to taste) (reserve remaining for garnish). Bring to a simmer, and cook, stirring occasionally, until sauce thickens, 3-4 minutes. Stir in steak strips and green onions, and cook until no pink remains on steak, 1-2 minutes.

Toss Chinese broccoli with steak strips, green onions, and sauce. Place a serving of rice on a plate. Add Mongolian beef and garnish with chopped peanuts and remaining red pepper flakes (to taste).