I came up with this recipe when I decided to become a Vegetarian. I was having a hard time finding meals that were filling. And this one is very filling. My husband and kids still eat meat. I put this as a side dish for them. Very yummy!
Preheat the oven to 425F
Combine the spices in a small bowl and set aside.
You can get a jump start on the onions while you slice the potatoes and tomatoes. Heat half of the olive oil, 2 tablespoons, in your largest skillet or dutch oven over high heat. When hot, stir in the onions along with a few pinches of salt. Cook for a few minutes, stirring often, until the onions soften up - 4-5 minutes. Turn the heat down to medium and stir in the butter. Stirring regularly, cook another 10 - 15 minutes at this temperature, or until the onions just begin to caramelize a bit. Dial the heat back a shade more, and cook until the onions are deeply golden, this might take another 20 minutes.
In the meantime, use a mandoline to slice the potatoes into 1/8-inch thick rounds. Place in a medium bowl along with the cream, and spices from the bowl. Also add in the garlic. Mix well and set aside.
Use a knife to cut the tomatoes into 1/4-inch thick slices. Arrange across a large plate and sprinkle with salt and some pepper. I also do a pinch of sugar to bring out the sweetness of the tomatoes.
Smear half the caramelized onions across the bottom of a gratin or baking dish. Take half of the potatoes and half of the tomatoes and arrange on top of the onion layer. Drizzle with a couple tablespoons of cream from the potatoes and a tablespoon of olive oil. Season the layer with a pinch of salt and half the basil.
Scatter the remaining onions across the potatoes and tomatoes already in the pan. Then arrange another layer of tomatoes and potatoes on top. This will be the top of your gratin, so do your best to make it look nice. Pour the remaining cream, from the potatoes, and last tablespoon of olive oil across the top. Season with another pinch of salt and the remaining basil. Gently press down on the vegetables so the cream comes up through the layers of vegetables evenly.
Cover tightly with aluminum foil and bake for 40 mins, or until the potatoes are completely tender throughout. Then remove the foil and up the oven to 450F to toast the top until the top takes on a nice golden color. Depending on your oven you might need to cook longer.