1. Scrub the potatoes well. Cook in boiling salted water in a medium-size saucepan 20 minutes, or until tender. Drain, then shake in pan over low heat just until skins start to burst.
2. While potatoes cook, remove stems from spinach; wash leaves; drain slightly. Pile into a large skillet; sprinkle lightly with salt and cover. (No need to add any water.)
3. Heat slowly until steam appears, then steam 3 minutes, or just until leaves wilt; drain well. Sprinkle with lemon juice; toss lightly.
4. Spoon the spinach onto heated serving plates. Peel the skin from potatoes; press each through ricer onto spinach on plates.