Drizzle each pesto-covered chicken breast with about 1 1/2 teaspoons of olive oil (you’ll use roughly 2 tablespoons total). This helps the chicken stay extra juicy and gives you a richer sauce in the bottom of each packet.
Fold the long sides of the foil up and over the chicken, then fold the edges together tightly to seal. Fold in the short ends as well so each piece of chicken is completely enclosed in its own packet. Make sure there are no big gaps where juices could leak out.
Arrange the foil packets seam-side up in the slow cooker crock, packing them in snugly in a single layer. It’s okay if the sides touch or overlap slightly; just avoid stacking them directly on top of each other if you can.
Cover the slow cooker with the lid and cook on LOW for 3 1/2 to 4 hours, or until the chicken is cooked through and reaches an internal temperature of 165°F. Cooking on LOW keeps the chicken tender and prevents it from drying out.
Once the cooking time is up, turn off the slow cooker. Carefully open the lid—this is when that incredible pesto aroma hits you—and use tongs to gently lift out each foil packet, setting them on plates or a tray.
Let the packets rest for a couple of minutes, then carefully open them, watching out for hot steam. Spoon the pesto juices from the bottom of each packet over the top of the chicken. Serve the chicken directly in the foil for easy cleanup, or transfer to plates and pour the juices over whatever side you’re serving.
