Pat the chicken breasts dry and slice thicker breasts in half horizontally to make even, thin pieces. Season both sides with salt and pepper.
Heat 2 Tbsp oil in a 9-inch skillet over medium heat until shimmering.
Sear the chicken in the skillet 2–3 minutes per side until golden; transfer to a plate and set aside.
Add the minced garlic to the skillet and sauté about 1 minute until fragrant.
Raise heat to medium-high and pour in the 2 cups chicken stock to deglaze the pan, scraping up browned bits; simmer until the stock reduces by about half, about 5–8 minutes.
Reduce heat to medium and stir in the heavy cream, cubed cream cheese, chopped sundried tomatoes, grated Parmesan, spinach, and lemon juice; stir constantly until the sauce bubbles and thickens.
Return the seared chicken to the skillet and simmer in the sauce until cooked through, about 5 minutes.
Taste and adjust seasoning with salt and pepper; garnish with freshly cracked black pepper before serving.
