For the pastry, put flour, polenta, and salt into a large bowl and stir together.

Rub butter into flour with your fingertips until the mixture resembles coarse bread crumbs.

Stir in Parmesan and thyme and add just enough egg to bind dough together.

Roll dough out on a lightly floured surface into a 10-inch circle and press into a 9-inch tart pan. Cover and chill for 45 minutes to allow dough to rest.

Preheat oven to 400°F. Bake tart shell for 15 minutes. Once removed from oven, reduce heat to 300°F.

For the tart, put Jerusalem artichokes, vinegar and 1 teaspoon salt into a medium pot and cover with water.

Bring to a boil, reduce heat and simmer, cooking it for about 15 to 20 minutes. (Don't let artichokes break up.)

Drain artichokes and pat them dry. Set aside.

Melt butter in a large skillet over medium heat. Add onions and cook until soft.

Add garlic and cook for about 1 more minute. Take off heat and drain well.

Mix the half-and-half, sour cream, Parmesan, egg yolks, and eggs in a small bowl and add to the onion mixture.

Season with remaining salt, pepper and nutmeg.

Arrange artichokes in an overlapping fashion in the tart shell.

Pour sour cream mixture over artichokes and bake for 40 to 45 minutes, or until filling is set. Serve tart warm or cold.