Dice onion. Mince garlic cloves. Thinly slice 1/2 cup drained oil-packed sun-dried tomatoes.

Cut chicken breasts in half horizontally. Pat dry with paper towels. Season with salt and pepper.

Heat 1 tablespoon of the olive oil in a skillet over medium-high heat. Add half of the chicken in a single layer and sear until browned, 3 to 5 minutes per side. Transfer to a large plate (the chicken will not be cooked through). Repeat browning the remaining chicken.

Reduce the heat to medium. Add the remaining 1 tablespoon olive oil, if needed, and heat until shimmering. Add the onion and cook, stirring occasionally until softened, 2 to 3 minutes. Add garlic, sun-dried tomatoes, and Italian seasoning. Cook until fragrant, 30 seconds to 1 minute. Add spinach and cook, stirring regularly, until just wilted, 1 to 2 minutes.

Add the Parmesan, heavy cream, and the remaining 1/2 teaspoon kosher salt. Bring to a simmer, scraping up any browned bits from the bottom of the pan with a wooden spoon. Reduce the heat to maintain a gentle simmer. Return the chicken and any accumulated juices on the plate to the pan and cook until the chicken is cooked through, 4 to 7 minutes.