Preheat the oven to 400 degrees F. Toss both kinds of potatoes, the parsnips, onion, garlic, olive oil, 1 teaspoon salt and 1/4teaspoon pepper in a bowl. Spread in a single layer on a baking sheet; bake until tender, about 45 minutes. Let cool slightly.
Meanwhile, mix 2 tablespoons water and the sugar in a skillet and bring to a boil over medium heat. Cook, swirling the pan, until amber, about 7 minutes. Remove from the heat and stir in the vinegar and 1/4teaspoon each salt and pepper. Pour the caramel into a 9-by-13-inch glass baking dish and spread with a rubber spatula. (See our notes below on our experience making the caramel.) Sprinkle the sage and oregano on top.
Arrange the roasted potatoes and parsnips in a single snug layer on top of the caramel. Scatter the onion and garlic over the roasted vegetables; sprinkle evenly with the mozzarella.
Roll out the puff pastry on a lightly floured surface into a 9-by-13-inch rectangle. Pierce the pastry all over with a fork, then lay it on top of the mozzarella, folding the edges under to fit, if necessary. Bake 20 minutes, then reduce the oven temperature to 350 degrees F and continue baking until the dough is cooked through, 15 to 20 more minutes.
Let the tart cool 10 minutes in the baking dish, then carefully invert it onto a cutting board. Replace any vegetables that stick to the dish, if necessary.
We had trouble making the caramel sauce, so we ended up just cooking the sugar down a minute or two only after it began to bubble, and poured that into the pan. We didn’t wait for it to get to a caramel state since the oven would likely caramelize it all it needed to be. A reviewer on Food Network had noted this option and we went with it, worked great.
The amount of veggies once cut seems like a lot, but remember this is making a really big tart, so use them all.
Run a knife around the edges to help loosen it once baked. Put a board or platter over the top of the pan and carefully but quickly flip the pan over and tap on the bottom a little to further help loosen the tart. If it doesn’t come out right away, don’t worry. Flip it back over if you have to and run the knife around again, etc. Sis blogging partner did this twice with ours and it came out almost perfectly after a little patience.
Further Note from Shillingford Organics:
Use other veggies too if you wish