Season both sides of the chicken with salt, pepper, and garlic powder.
Heat olive oil in a large skillet over medium-high heat.
Add the chicken and cook for 6–12 minutes depending on thickness, flipping halfway through until fully cooked. The internal temperature should reach 165°F.
Transfer the cooked chicken to a plate and set aside.
Add butter and chopped onions to the same skillet. Stir the onions around the pan to pick up the flavorful browned bits from the chicken.
Reduce heat to medium-low and cook onions for 12–15 minutes, stirring occasionally until deeply golden and soft.
Add fresh thyme, red pepper flakes, sugar, and balsamic vinegar. Stir and let the vinegar cook down for 1–2 minutes.
Slowly whisk in the chicken broth to deglaze the skillet.
Increase heat to high and simmer the sauce for 2–3 minutes until slightly reduced.
Lower the heat and slowly stir in the heavy cream. Allow the sauce to come to a gentle simmer without boiling.
Return the chicken to the skillet and spoon the Skillet Chicken in Balsamic Caramelized Onion Cream Sauce over the top.
Serve immediately while warm.