Season the chicken breast evenly on both sides with sea salt, black pepper, and Italian seasoning.
Heat the olive oil in a large skillet over medium-high heat until shimmering.
Place the chicken in the skillet and sear for 5-6 minutes per side until golden brown and cooked through.
Remove the chicken from the skillet and set aside on a plate.
In the same skillet, add the diced onion and minced garlic, sautéing for 2 minutes until translucent and aromatic.
Add the cherry tomatoes to the pan and cook for 3-4 minutes, stirring occasionally, until the skins begin to burst.
Deglaze the pan with balsamic vinegar, scraping up any browned bits from the bottom.
Add the fresh spinach and stir continuously for 1 minute until the leaves are just wilted.
Return the chicken and any resting juices to the skillet, spooning the tomato mixture over the meat.
Sprinkle with parmesan cheese and serve immediately while hot.