Place the yellow squash on a large cutting board, trim off the ends with a chef’s knife, then slice them into thin rounds about ⅛–¼ inch thick; set the slices in a pile.

Slice the tomatoes on the cutting board into similar ⅛–¼ inch rounds with the chef’s knife, keeping the slices as even as possible and setting them aside gently so they hold their shape.

Peel the potatoes on the cutting board if you like, then slice them into ⅛–¼ inch rounds with the chef’s knife or a mandoline, stacking the slices in a neat pile.

Put the potato and squash slices into a large mixing bowl, leaving the tomato slices aside for now.

Add 3 tbsp olive oil, the minced garlic, 1 tsp thyme, ½ tsp rosemary, 1½ tsp kosher salt, and ½ tsp pepper to the bowl with measuring spoons, then toss everything with your hands or a spoon until the slices are lightly coated.

Gently season the tomato slices on the cutting board by sprinkling a small pinch of salt and pepper over them so they have some flavor before layering.

Lightly oil the bottom of a 9×13 inch baking dish with a small drizzle of olive oil, then spread it around with your fingers or a piece of paper towel.

Create small stacks on the cutting board by alternating a slice of potato, a slice of squash, and a slice of tomato, then stand the stacks upright in the baking dish so the slices lean on each other like tiles.

Continue stacking and arranging the vegetables in tight rows until the dish is filled, tucking in extra pieces where needed so there are few gaps.

Drizzle any remaining seasoned oil and garlic from the mixing bowl over the vegetables in the baking dish with a spoon so it falls between the slices.

Cover the baking dish loosely with aluminum foil, sealing the edges lightly with your hands so steam stays inside.

Place the covered dish on the middle rack of a preheated 375°F oven using oven mitts and bake for 30 minutes until the potatoes start to soften.

Remove the foil with oven mitts, then, if using, sprinkle ¼ cup grated Parmesan and 1 tbsp olive oil evenly over the top of the tian with a spoon.

Return the uncovered dish to the oven and bake at 375°F for 15 minutes until the potatoes are fully tender when pierced with the tip of a knife and the top has light golden spots.

Take the baking dish out with oven mitts and let the tian rest for 5 minutes so the juices settle, then use a spoon or spatula to lift out portions, keeping the slices stacked together as much as possible, and serve warm as a side dish.