Pat your chicken breasts dry with paper towels—this is the step that determines whether your chicken develops proper color or steams itself pale. I learned this the hard way after making gray chicken for years. Heat olive oil in your largest skillet over medium-high heat until the surface shimmers, which takes about two minutes.
Season both sides of the chicken with salt and black pepper. Place breasts in the hot pan and don't touch them for four full minutes because moving them breaks the crust that's forming. You'll hear a distinct sizzle when they hit the oil; that sound means the Maillard reaction is happening, which creates flavor depth nothing else can provide. Flip and cook the other side for three more minutes until golden.
Remove chicken to a plate—it won't be fully cooked yet, and that's exactly right. Reduce heat to medium, add diced onion to the same pan, and cook for three minutes until the edges turn translucent. I stir constantly here because burnt onions taste bitter, which ruins the creamy tuscan chicken cozy entirely.
Add minced garlic and cook for exactly one minute, stirring throughout—garlic burns in seconds and becomes harsh and acrid. Stir in sun-dried tomatoes, oregano, and basil, then pour in chicken broth. Return chicken to the pan and simmer for 12 minutes covered at medium heat. I always peek after eight minutes to check that nothing's sticking, though it shouldn't be.
Remove the lid and stir in heavy cream and mozzarella cheese, stirring until the cheese disappears completely. The sauce will turn pale and smooth. Cook uncovered for two more minutes at medium heat, watching for gentle bubbles around the edges—this isn't a boil; it's a whisper-simmer because high heat breaks cream-based sauces.
Add fresh spinach leaves and cook for exactly two minutes, stirring twice. The spinach wilts down and releases its mineral flavor into the sauce. This late addition keeps the spinach bright and textured rather than gray and tired. Pour lemon juice over everything and taste—you might add another pinch of salt, and that's fine.