1. In a pot of boiling salted water, cook the Jerusalem artichokes for 15 minutes. Drain and plunge into cold water for 3 minutes.
2. Slice the Jerusalem artichokes into rounds.
3. In a pot of cold water, place the beets. Bring to a boil and cook the beets for 25 minutes. Drain and place in ice water for 3 minutes. Drain.
4. Peel the beets and slice into rounds. Reserve.
5. In a small bowl, mix the ricotta, salt, and pepper. Add the lemon juice and orange juice; mix.
6. Chop half of the cornichons and add to the ricotta.
7. Arrange the spinach on plates. Top with beet and Jerusalem artichoke slices. Top with the ricotta mixture. Garnish with cornichons. Serve immediately.