Preheat oven to 375 F (190 C).
Start by preparing your onions. Peel all three onions and thinly slice one of the onions into long thin strips. Slice the other two into 8 wedges each by cutting the onion in half lengthwise, in half again, and then again. Try to keep the wedges of onion intact.
Heat a medium sized skillet over medium to high heat. Add 1 teaspoon of butter. Begin to brown the butter by letting it completely melt and start to bubble and foam.
Once the butter has browned, add the minced garlic to the skillet. (You will know the butter is browned when the bubbles begin to disappear, the milk solids have dropped to the bottom and turned a golden to dark brown and there is a nutty aroma in the air.) Using a spatula to stir occasionally to ensure that the garlic does not burn, cook until fragrant, about 30-60 seconds.
Remove the thyme leaves from the stem and add them along with the thinly sliced onions to the skillet. Reduce the heat to medium and saute them until the onions are translucent, about 2 minutes. Add in the red wine and beef stock. Increase the heat to bring the liquids to a boil. Once boiling, lower the heat to a simmer and reduce until one fourth of the liquid remains. Stir the onions around occasionally. Once reduced, turn off the heat and mix in the balsamic glaze and reserve on the side.
While the wine and stock are reducing, heat a large cast iron skillet. Add 2 tablespoons of butter and brown it, as you did above. Turn the heat off and sprinkle the granulated sugar over the skillet. Add the in the onion wedges, with one side of the wedge face down, in a single layer. You should be able to almost fill the surface of the skillet with a single layer of wedges. Turn the heat on to medium high and cook, undisturbed, around 3-5 minutes or until golden. Using a spatula, tongs or fork, carefully flip each onion wedge. Cook for another 2-3 minutes and turn off the heat.
In an even layer, top the wedges with the reserved caramelized thinly sliced onions. The caramelized onions should also fall into the gaps between the wedges. Sprinkle with gruyere cheese.
Roll out the cold pie crust into a circle just smaller than the size of your cast iron on a lightly floured surface. Carefully lift up the dough and place it on top of the onions and cheese in the cast iron skillet. If it ok if it is not perfect, patch up any small holes by pinching it together with your fingers. The crust should be sitting close to the edges of the cast iron.
Place the skillet in the oven and bake until the crust is golden, around 15-20 minutes. Remove the skillet from the oven, let it sit and cool for 20-30 minutes. To flip the tarte tatin out of the cast iron, hold a plate or cutting board that is larger than the cast iron on top of the skillet. Holding the plate or board tightly and the skillet in the other hand, quickly but carefully flip the cast iron over. Lift the cast iron up, leaving the tart on the board or plate. If any pieces of onion got stuck to the skillet, simply scrape them out and place them back onto the tart. Enjoy warm or at room temperature.