Melt 2 T. butter in large skillet over medium heat. Stir in onions. As soon as you start to see a little browning, reduce heat to low. Add salt & pepper. Cook, stirring occasionally, until caramelized. This takes 40-45 minutes. Don't try to rush it!! It's a low-and-slow process! Dissolve the Better Than Bouillon in the hot water.

While the onions are cooking, slice the mushrooms, strip the thyme leaves from the stems, and mince the garlic. Frequently check and stir the onions.

When the onions have beautifully caramelized, stir in the sherry. Cook for 1 minute and add the beef broth. Stir and cook another 2 minutes to slightly reduce the liquid. Remove from skillet into a bowl and set aside to cool.

Add 2 T. butter to the skillet. Stir in the mushrooms, salt and pepper, and cook over medium-high heat, stirring frequently, for 5-8 minutes. Grate the cheese.

When mushrooms have browned, stir in the garlic and thyme and cook until fragrant, about 2 minutes. I know they're ready when my sister yells from the other room, "OMG! what smells so good?!!" 😉😋 Remove from the skillet and stir them into the bowl of cooling onions. I put the bowl in the fridge because I do all this prep early in the day then make the tart in the evening.

With a sharp knife, gently score the pastry about 1/2 inch from the edge, all the way around, taking care not to cut all the way through the pastry. Use a fork to prick the center of the puff pastry a few times (not the border) to help it bake flat. (Place the pastry into the fridge to chill for a few minutes if it is becoming warm.) Egg wash the 1/2 inch border of the tart.

Sprinkle 3/4 cup of the cheese on the bottom, then top with the mushrooms and caramelized onions. Top with the steak slices, trying to distribute evenly. Sprinkle over this another 1/2 cup of cheese.

Bake for 25 to 30 minutes (mine took about 28 minutes), or until a deep, golden brown. Allow to cool on the baking sheet for 10 minutes, then sprinkle over the remaining thyme leaves as garnish. Cut the tart into pieces. Serve hot, or at room temp.