This is a summer take on a winter favorite. Potato gratin seems much too heavy to enjoy in the warm summer months but if you swap the starchy potatoes for bright cherry tomatoes and summer squash, it suddenly becomes a much lighter dish. Now you might be wondering what qualifies as "summer squash" and if so, here are the heavy hitters: zucchini, pattypans, and yellow crookneck. I also add a bit of sweet onion (walla walla, vidalia, or maui) between the layers.
You can make this recipe in little individual gratin dishes or opt instead for a larger family-style vessel; I promise it will be super delish either way. Start by simmering some heavy whipping cream (a cup or so for 2-3 servings of gratin) in a saucepan with a couple of garlic cloves, a sprig of fresh thyme, 2 cloves, and 1/4 of a sweet onion. You want to impart lots of flavor into your cream. Let it slowly simmer while you shave your veggies. I use a mandolin to ensure everything is the same thickness and therefore will cook at the same rate, nice and evenly. You don't have to have a big fancy mandolin, I usually use a little handheld one that I bough for $1 somewhere in the international district of Seattle (see below).
Next layer the squash, onions, and tomato in your baking dish(es) stopping between each layer to sprinkle a bit of sea salt, cracked pepper, and good quality parmesan cheese (if you are feeling indulgent you can add a bit of gruyere too!). Keep layering until you reach the top of your dish. Next, strain your cream and pour enough cream into the baking dish so that it reaches halfway up the side. The squash has a lot of water weight, so you don't need too much cream as liquid will seep out of squash as soon as you start to bake it. Finish by sprinkling a bit of cheese on the top along with a few nubs of butter. Bake at 400 for 30-45 minutes or until golden brown and bubbly on top. Pull from oven and let cool 5 minutes before enjoying!