Remove and discard giblets and lumps of fat from chicken. Rinse bird inside and out; pat dry. If desired, fold wing tips under first joint and tie drumstick ends together. Set breast side up on a V-shaped rack in an 11- by 17-inch roasting pan. Squeeze juice from half of 1 lemon, reserving remaining half. Thinly slice other lemon.
Starting at the neck, gently ease your fingers under chicken skin to loosen it over breast. Push 4 to 6 lemon slices and 6 to 8 sage leaves under skin. Place remaining lemon slices, reserved lemon half, and remaining sage leaves in body cavity.
In a small bowl, mix 1 tablespoon of the lemon juice, olive oil, paprika, cumin, salt, and pepper. Rub about half this mixture evenly over chicken.
In a bowl, mix potatoes and shallots with remaining paprika mixture until well combined. Distribute vegetables around chicken in pan.
Roast in a 425° oven, stirring vegetables after about 45 minutes (add 1/4 cup water if pan begins to scorch), until potatoes are tender when pierced and a thermometer reaches 170° when inserted through the thickest part of breast to the bone and 180° when inserted through thickest part of thigh at joint, 1 1/4 to 1 1/2 hours. (If chicken is done before potatoes, return pan with vegetables to oven and continue to cook while chicken rests in step 6.)
With a carving fork, lift bird and tilt to drain juices into pan. Set chicken on a rimmed platter. Arrange vegetables around chicken. Let rest in a warm place about 15 minutes. Carve chicken and serve.