Squash, potato, and caramelized onion tart

Directions

• Preheat the oven to 350°F.
• Start to caramelize onions: heat a cast iron skillet over medium heat and add a few tablespoons of butter. Add the thinly sliced onions and a good pinch of salt. Stirring every so often. Once they become translucent and gain some colour, lower heat to medium low. Continue to stir every so often as you continue the recipe.
• Melt 1 stick (113g) of butter in a pan. Scrape pan clean, and set butter aside to cool.
• Assemble crust: combine the cornmeal, flour, grated parmesan, salt, sugar black pepper. Then pour in the melted better and stir to combine. 
• Press into an 11 inch tart pan, with a removable bottom. Focusing first on the sides and then using what is left to cover the Bottom. You can use the flat bottom of a measuring cup to help even the layer. 
• Prick the bottom of the crust with a fork every inch or so. Then place tart pan on a baking sheet, and bake in the oven for 15 minutes.
• Check on your onions: At this point your onions should look jammy and brown. Add a few splashes of wine, stock or even water to lose up the mixture. Cook on low heat for a few minutes then remove from heat. 
• Prep the vegetables: Peel the honey nut squash and potatoes, delicate squash does not need to be peeled! Cut vegetables in half lengthwise and then thinly slice into half moons. If you have a mandolin this is a great time to use it! You can them quite thin!
• Spread the caramelized onions on the baked tart crust. NOTE: either do this in the sheet pan the crust was baking in, or be very careful moving it, remember it is a removable bottom!
• Evenly sprinkle on the grated cheese and half of the fresh thyme leaves over the caramelized onions.
• Now begin to arrange the vegetables in concentric circles, starting at the outer edge and working towards the middle. 
• Brush the squash and potatoes generously with the 1/2 stick of melted butter. Season all over with kosher salt.
• Return tart to the 350 F oven and bake for 1 hour. After 1 hour, remove from the oven and lightly sprinkle with a bit of sugar, this will help caramelize the edges. Then return to the oven for 20 minutes.
• Remove from oven, sprinkle with the remaining thyme leaves. Let cool for 15 minutes, then carefully remove from tart pan.