1. Preheat oven to 400° F.
2. Heat 2 tablespoons of the olive oil in a 12-inch oven-proof skillet over medium-low heat. Add the sliced onion, and cook, stirring occasionally, until onions are soft and start to take on a golden color, 10-12 minutes.
3. While the onion cooks, thinly slice the potatoes and parsnip. (If you have a food processor with a slicing attachment or a mandolin, this is the preferred method, if not, use a chef’s knife.) Place into a large bowl and season with the salt, pepper, and thyme.
4. Add the cooked onions to the vegetables and toss to combine with the seasonings.
5. Layer the sliced vegetables in a circular design in the skillet, alternating the potatoes and parsnip. Lay as flat as possible, pressing down as you go.
6. Cut the 2 tablespoons of butter or buttery sticks into little cubes and dot the top of the vegetables.
7. Cut a piece of parchment paper into a round about the same size as the skillet and place on top of the vegetables, pushing down to compress vegetables, and lightly coat the parchment with a drizzle of olive oil to prevent burning.
8. Bake at 400° F on the top rack for 45 minutes.
9. When finished baking, run a knife around the edge of the tart to release it from the skillet and allow it to cool slightly before slicing. Sprinkle with fresh thyme as a garnish, if desired.
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