Preheat a large skillet over medium-high heat. Season chicken with salt and pepper.
Add olive oil and a spritz of cooking spray to the pan. Swirl oil around to coat. Add chicken and sear until golden brown and mostly cooked through, about 4-5 minutes per side. Remove chicken from pan and set aside.
Lower heat to medium. Add garlic and cook until just fragrant, about 30 seconds. Add chicken stock in to deglaze, scraping up any browned bits from the pan. Let simmer for 1-2 minutes.
In a small bowl, whisk together almond milk, half and half, and cornstarch. Add this mixture to the pan and bring to a simmer.
Mix basil, oregano, and sundried tomatoes into the sauce. Continue to simmer until slightly thickened, about 5 minutes.
Add parmesan and spinach. Stir until cheese is melted into the sauce and spinach is just wilted. Place chicken and juices back in the skillet and simmer until chicken is heated through. Taste sauce for seasoning and add salt and pepper to taste.
Let rest for 5 minutes off the heat, so chicken can rest and the sauce can thicken a bit more.