Preheat the oven to 350°F.

Heat a cast iron skillet over medium heat and add some oil, onions and a generous pinch of salt. Stir occasionally. Once translucent and beginning to brown, lower heat to medium-low. Stir occasionally as you proceed with the recipe.

Melt 1 stick of butter, set aside to cool.

Combine the cornmeal, flour, grated parmesan, salt, sugar, and black pepper. Pour in the melted butter and stir to combine.

Press the mixture into an 11-inch tart pan with a removable bottom, focusing on the sides first and then cover the bottom.

Prick the bottom of the crust with a fork every inch or so. Place the tart pan on a baking sheet, bake in the oven for 15 minutes.

Check the onions: they should look jammy and brown. Add a few splashes of wine, stock, or water to loosen the mixture. Cook on low for a few minutes, then remove from heat.

Peel the honeynut squash and potatoes. Cut the vegetables in half lengthwise and thinly slice them into half-moons. Use a mandoline if you have one.

Spread the caramelized onions over the baked tart crust, then the grated cheese and half of the fresh thyme.

Arrange the squash and potatoes in concentric circles, starting from the outer edge and working towards the middle.

Brush generously with the melted butter. Season with kosher salt.

Bake at 350°F for 1 hour. After 1 hour, remove and lightly sprinkle with sugar. Then return to the oven for another 20 minutes.

Let it cool & carefully remove from tart pan.