Slice 1 small yellow onion (about ⅔ cup). Mince 3 garlic cloves. Thinly slice ½ cup drained oil-packed sun-dried tomatoes. Finely grate 1 ounce Parmesan cheese if needed (about ½ cup lightly packed), or measure out ⅓ cup store-bought grated.

Cut 2 large boneless, skinless chicken breasts in half horizontally (also known as butterflying). — or if using thighs, do not butterfly. Pat dry with paper towels. Season all over with olive oil, 1 teaspoon of the kosher salt and ½ teaspoon black pepper.

Heat 1 tbsp olive oil in a large skillet over medium heat until shimmering. Add the onion, stirring occasionally until caramelized, 45 minutes to 1 hour

Heat grill up to 450°. Cook chicken for 6 minutes on each side. Transfer to large plate. OR:

Heat 1 tablespoon of olive oil in another large skillet over medium-high heat until shimmering. 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). Add 1 tablespoon of the olive oil to the pan and repeat browning the remaining chicken. Transfer to the same plate.

Add the garlic, sun-dried tomatoes, and 2 teaspoons dried Italian seasoning to the onions. Cook until fragrant, 30 seconds to 1 minute. Add 5 ounces baby spinach a handful at a time. Cook, stirring regularly, until just wilted, 1 to 2 minutes.

Add the Parmesan, 1 ¼ cups heavy cream, and the remaining ½ 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 and registers at least 165ºF on an instant-read thermometer, 4 to 7 minutes.