1. Toast pine nuts in a small dry skillet over medium heat, stirring, until golden and fragrant, about 5 minutes. Transfer to a food processor and let cool slightly.
2. Preheat oven to 375°F. Add spinach, goat cheese and lemon zest to food processor and pulse with pine nuts until mixture is smooth. Scrape sides of bowl as necessary. Taste and season with salt and pepper. (Makes about 1 cup filling.)
3. Slide your fingers under the skin of each chicken breast, loosening skin to form a pocket between the skin and meat. Stuff 2 to 3 Tbsp. filling under skin, gently pushing as much of the filling as you can into the far corners without tearing skin. (You will have about 1/3 cup extra filling; discard.)
4. Mist with cooking spray a baking dish large enough to hold chicken in one layer with space between pieces. Place chicken filling-side up in dish and drizzle with olive oil. Season with salt and pepper. Bake until skin is lightly browned and chicken is cooked through (an instant-read thermometer placed in thickest part should read 160°F, or cut into thickest part of a breast to check), 40 to 45 minutes. Serve immediately.