1mediumonion, about 7-8 ounces, peeled, ends removed
1 ½teaspoonsalt, divided
1cuphalf and half
1cupgrated Cheddar cheese
Get the pie crust ready
On a lightly floured surface, roll out the half-portion of All-Butter Pie Crust into a 13"-inch circle. Fit the dough circle into a 10-inch tart pan, and gently press the dough into place all around the pan and against the sides. Fold the edge under itself and crimp it firmly all the way around.
Cover the shell with plastic wrap and place the pan in the fridge until the filling is ready.
Roast the carrots and onions
Preheat the oven to 400° F. Line a large baking sheet with parchment paper.
Peel the carrots and cut off the tops and ends. Cut the carrots into 1 ½-inch chunks. Place them in a medium bowl.
Cut the onions in half, then cut each half into three or four chunks. Place these in the bowl as well. Add to the bowl the oil, 1/2 teaspoon of the salt and the pepper. Stir to coat the vegetables with the oil and seasonings, then spread the veggies on the prepared sheet into an even layer.
Place the pan in the oven and roast the veggies 35 to 40 minutes, until the carrots are soft. Remove the pan from the oven. Increase the oven temperature to 425° Move the middle oven rack down one spot so that it's closer to the bottom of the oven.
Prepare the filling
Allow the roasted carrots and onions to cool a little, and then transfer them to the bowl of a food processor, along with the 1 cup of milk. Blend these until they are thoroughly combined and smooth. Set this mixture aside. (An immersion blender with a tall container also works well for this step.)
In a large bowl beat the eggs together with the half and half, the remaining teaspoon of salt and the thyme. Add the carrot mixture to the bowl and whisk to combine it all together. Stir in half of the Cheddar cheese.
Fill and bake the tart
Take the pastry shell from the fridge and remove the plastic. Pour carrot filling into shell. Sprinkle the remaining Cheddar cheese evenly over the top. Place the tart carefully into the oven on the lowered rack.
Bake the tart for 15 minutes. Reduce the temperature to 325° F and bake for another 20 minutes, until a knife inserted into the center comes out clean.
Place the tart on a cooling rack and allow it to cool for at least 15 minutes before slicing and serving.