Add the flour, salt, sugar and rosemary to the bowl of a food processor and pulse a few times to combine. Then add the cold cubed butter and pulse until it becomes a coarse meal. Then add the vinegar and the starter and pulse a few times until the dough just starts to come together with some crumble still.

Pour the dough out onto a clean work surface and press the dough together until it forms a disc. Wrap it in plastic wrap and set in the fridge to chill for at least an hour. (Roast the squash while the dough is chilling.)

After the dough has chilled, roll it out between two pieces of parchment paper to a little over 13 inches in diameter and about an 1/8th of an inch thick then carefully place it over the tart pan and press it into the sides and corners. Then use a rolling pin to roll over the surface of the tart pan to trim off the any edges hanging over the sides. Dock the bottom of the tart all over the bottom with the tines of a fork and set in the freezer to chill for 15 minutes.

Then pre-bake the docked tart shell in the 350-degree oven for 10 minutes. Remove and let cool on a wire rack

Preheat the oven to 375F/190C.

Heat 2 tablespoons oil in a large saute pan over medium heat and add all the sliced onions and stir to coat the onions with the oil. Spread them out evenly over the pan and let cook. Reduce the heat to medium-low and cook, stirring occasionally, for a total of about 20-25 minutes until they are nice and dark blonde and deep purple-colored. Then add the minced garlic and cook a minute or so more until fragrant. Set aside.

Then toss the sliced butternut squash, yam and purple sweet potato with the 2 tablespoons of oil, chopped rosemary and a pinch of salt and pepper.

In a medium bowl stir together the shredded gouda, ricotta, parmesan and salt and pepper. Then fold in the slightly cooled caramelized onions.

Line the bottom of the pre-baked tart shell with about a third of the onion/ cheese mixture and spread evenly. Then lay out a layer of the squash and sweet potato slices. Then repeat with two more layers of the cheese mixture and two more of the squash, ending with the squash. Drizzle a tiny bit of olive oil over the top of the tart.

Bake until golden brown and the squash is tender, about 1 hour- 1 hr 15 minutes.

While the tart is baking, coat the bottom of the pan you used for the onions with extra-virgin olive oil. Then heat until shimmering. Add sage leaves in a single layer and fry for about 30 seconds to a minute. Remove with a slotted spoon and transfer to a plate lined with paper towels and sprinkle with a little kosher salt.

Let the tart cool for about 10-15 minutes, top with the fried sage leaves and enjoy.