One of the best, most useful, things my wonderful cook of a mother taught me as I was growing up was how to think in substitutions. I know that sounds like an odd statement but I have learned in my adulthood that not every home cook has been given the benefit of those skills. I know many wonderful cooks who, if they want to make a recipe and don't have certain ingredients they go to the store for those specific ingredients without any thought of "Hmmm...what do I have that would work, put my pantry or fridge supples in use, and be more economical than purchasing new items?"
It doesn't always work but in many cases, it does. For example, in many chicken, rice, and vegetable casseroles that call for, for example, fresh asparagus, if that's not in season, then 99% of the time, either broccoli or fresh green beans will not only work, but be delicious too. In the example of this recipe, the red potatoes would make an excellent tart but what I had on hand were Yukon yellows, and they too were delicious. For cheeses, this recipe calls for gruyere, feta, and mozzarella, so we need something with a bit of a tangy, something creamy, and some binding. I had asiago (tangy), feta, and provolone (binding) available, and again, they were delicious in this recipe. I wish I had had fresh thyme available, but having only dried at the time, it's a simple matter of using half what the recipe calls for since dried herbs tend to be more intensely flavored (unless they're a million years old).
The next time I make it I may have the red potatoes in the house to use and maybe the fresh thyme too...or perhaps I'll try a little tarragon or rosemary, which would also be fabulous. I love the combination of grilled onions, potatoes, and herbs: the dinner trifecta!