Spray the slow cooker with cooking spray. In a small bowl, mash the butter with half the lemon juice, the 2 shallots and the parsley until combined.
Spread 2 tablespoons of the butter mixture on the bottom of the cooker. Arrange the potato slices on top. Season with salt and pepper. Layer the carrot slices on the potatoes. Season with salt and pepper.
Using your fingertips, gently loosen the skin from the turkey breast, leaving it attached on one side. Spread the remaining lemon butter and the sage leaves under the skin with your fingers. Season the breast on both sides with salt and pepper. Place the breast, skin side up, on the vegetables. Drizzle the remaining lemon juice on top.
Cover and cook on high until an instant-read thermometer registers 170 degrees in the thickest part of the breast and the vegetables are tender when pierced, 3 to 4 hours. Transfer the turkey to a platter and let stand for 5 minutes, then slice. Spoon the vegetables onto plates and serve the turkey alongside.