While I 100% recommend the use of fresh truffles here (and I like how the summer ones are a little more floral and less earthy), a good drizzle of truffle oil would do in a pinch. If you're using oil, save it for finishing so that the flavour doesn't all cook off.
1 1/2 cups flour
1/4 tsp salt
1/2 tsp sugar
1/2 cup cold unsalted butter, cubed
2-4 tbsp ice water or very very cold milk
2 tbsp flour
2 tbsp unsalted butter
1 1/2 cups whole milk
1/2 cup grated gruyere cheese
1 large summer truffle (about 20 grams) or truffle oil
3 Roma tomatos, sliced width-wise 1/4 inch thick
1 medium zucchini, cut on a bias 1/8 inch thick
Make the pastry in a food processor and your life will never be the same again! Combine flour, salt, and sugar in a food processor and pulse to combine. Scatter cubes of butter on top of flour and process until you reach a consistent crumbly texture. Depending on how cold your butter is (it should be very cold!) about 10 seconds. Drizzle water over the top and process another 15 seconds so that the dough comes together. You shouldn't have any obviously dry patches and when you lightly pinch some between your fingers it should hold together easily. Once it does that, STOP!
Form your dough into a disk by dumping it out on a piece of plastic wrap and using the plastic to push it together. The less you touch it with your hands and warm it up at this point, the better. Work quickly and get it in the fridge for at least 1 hour. You want to give the gluten a chance to de-stress.
While that's happening, slice your veggies and roast them in a 450 oven to get rid of some of their moisture and add a depth of flavour. The tomatoes will take a little longer than the zucchini, but should be about 10 minutes max. You don't want to add much colour here.
And make a delicious, delicious sauce for the bottom. Combine flour and butter in a heavy-bottom pot set over medium. Cook until they are well combined and have just started to turn light brown and smell a little nutty. At this point add in all of the milk and start whisking. Keep going (whisking slowly) until the clumps are gone and the sauce has thickened to about pudding consistency. Shave in your whole truffle and season with salt and pepper.
Your oven should still be at 450. Or set it back there.
Roll out your pastry to about 1/8 inch thickness and flip it over the rolling pin for easy transfer. Fit into a tart pan, pushing the corners in to form a 90 degree angle. Trim excess just slightly above the tart rim (it will shrink a little bit during baking). Return to the fridge to chill out for 30 minutes. Pour yourself a glass of wine.
Spoon your bechamel into the tart shell and spread it out evenly. Top with a layer of tomatoes, arranged in concentric circles. The next layer is half of the cheese! Top with concentric circles of zucchini and finish with the rest of the cheese.
Put that baby in the oven and immediately lower the temperature to 400. Bake for 45-55 minutes, or until everything is brown, bubbly, and veggies are soft. Cool for 20 minutes before serving. Don't forget to save some for hand-held breakfast the next day!