Butter a large oven-proof dish; choose one with more surface area rather than a deep one.
Peel and chop the garlic; set aside.
Peel and slice the parsnips; set aside.
Wash the Jerusalem artichokes. Add them unpeeled to the boiling water, lower to a simmer, and cook for 15 minutes until just tender. Remove them from the water with a slotted spoon and set aside to cool.
Add the parsnips to the pot of water, and let them cook for about 10 minutes or until just tender. Drain.
When the Jerusalem artichokes are cool enough to touch, peel and slice them.
Arrange the parsnip and Jerusalem artichoke slices in the buttered casserole dish.
Start the oven heating to 200 C / 400 F / Gas Mark 6.
Bake for 15 minutes. Remove from oven and remove the foil. Pour the cream evenly over top. Return to the oven uncovered for a final 15 to 20 minutes, or until golden brown.