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!
1 large onion, sliced somewhat thickly ( a tad more than 1/4 inch thick)
1 lb. waxy red potatoes, washed and very thinly sliced (the blade on a food processor works perfectly for this)
2 tablespoons olive oil
1/2 cup grated gruyère
1/2 cup crumbled feta
1 cup grated mozzarella
sea salt and cracked black pepper
Preheat oven to 400°. Melt the butter (or olive oil) in a 9-10 inch ovenproof frying pan (cast iron is great) over medium heat. Add the thyme and onion and cook for 5 minutes.
Cook's note: Be SURE to get the butter or olive oil EVERYWHERE in this pan including the sides before laying the onion in or you'll have trouble flipping the tart out at the end. Trust me: be generous with the oil on this dish!)
Place the potato, oil, cheeses, salt and pepper in a bowl and toss to combine.
Top the onion with the potato mixture and bake for 45 minutes or until golden brown and cooked through.
Turn upside down to serve. The trick to this flip is to run a hard spatula or knife around the edges, lifting up a bit to make sure the potatos on the very edges will release when you tip the pan over. Even with that, don't be surprised or dismayed if a few pieces of onion stick to the pan. You can leave them or lift them out with a spatula and flip onto the top of the tart by hand and no one will be the wiser.
Cook's Note: Again, one of the great things about this recipe is that it's extremely customizable. For my dish, I used russet potatoes because that's what I had on hand, as well as asiago in place of the gruyere and provolone (1/2 cup only) in place of the mozzarella, again, because that's what I had on hand. I also added a clove of minced garlic in with the potatoes.