Heat olive oil in a large skillet over medium-high heat.
Season chicken cubes with salt and pepper.
Add chicken to the skillet and cook for 5 to 7 minutes until golden brown and cooked through.
Remove chicken from the skillet and set aside on a plate.
Add diced onion to the same skillet and sauté for 3 minutes until translucent.
Add minced garlic and red pepper flakes, cooking for 1 minute until fragrant.
Pour in chicken broth to deglaze the pan, scraping up any browned bits from the bottom.
Add spinach in batches, tossing frequently until just wilted.
Return the chicken to the skillet and drizzle with lemon juice.
Toss all ingredients together to combine and serve immediately.