In a medium bowl, toss the cubed chicken breast with cornstarch and black pepper until evenly coated.
Heat olive oil in a large skillet or wok over medium-high heat.
Add the chicken to the skillet and cook until golden brown and internal temperature reaches 165°F (74°C), approximately 7 minutes. Remove chicken and set aside.
Reduce heat to medium and add sesame oil to the same skillet. Stir in the minced garlic and grated ginger, cooking for 60 seconds until fragrant.
Add the broccoli florets and two tablespoons of water to the skillet. Cover with a lid and steam for 3 minutes until tender-crisp.
Return the chicken to the skillet and pour in the soy sauce and red pepper flakes.
Add the baby spinach and toss continuously until the leaves are just wilted, approximately 1 to 2 minutes.
Remove from heat and serve immediately.