there are a lot of IP [instant pot] au gratin recipes ... see if one of those might be close

There are a lot of variables and a zillion ways of making it. What I can say is that if it was in a restaurant, it probably had a generous amount of salt beside what's in the cheese, and probably was built on a roux made with cream rather than milk. But it might be something else entirely. Both the salt and cream will pump up the flavor. Not heart healthy, but other than that..

A gratin Dauphinois is as follows. No cheese!

Slice potatoes and put in a saucepan. Add a crushed garlic clove, salt and pepper and cover with milk. Simmer gently for 10 to 15 minutes.

Tip into an oven dish, pour over some cream and bake in the oven for about 40 minutes. It will be rich and creamy.

Islay

Try this one but substitute white cheddar for the Comte

https://leitesculinaria.com/61721/recipes-old-fashioned-potato-gratin.html

Note in the reviews, one reviewer adds extra cheese.

This is one of my absolute favorite dishes. Not exactly potatoes au gratin because of the addition of the ham.

https://leitesculinaria.com/223178/recipes-ham-cheese-potato-casserole.html

This is a favorite recipe for scalloped potatoes that I've tweaked by adding sharp white Cheddar cheese to the layers along with the Parmesan (and also on top). As plllog mentioned, the seasoning and cream really factor in. Potatoes need salt, for sure.

Scalloped Potato Gratin

1 3/4 cups heavy cream
1 sprig fresh thyme
2 garlic cloves, chopped
1/2 teaspoon ground nutmeg
Butter
2 pounds russet potatoes, peeled and cut into 1/8-inch thick slices
Salt and freshly ground black pepper
1/2 to 3/4 cup grated Parmesan, plus more for topping

Preheat the oven to 375 degrees F.

In a saucepan, heat up the cream with a sprig of thyme, chopped garlic and nutmeg.

While cream is heating up, butter a casserole dish. Place a layer of potato in an overlapping pattern and season well with salt and pepper. Remove cream from heat, discard thyme sprig, then pour a little over the potatoes. Top with some grated Parmesan. Make 2 more layers. Cover with foil lifting one corner to vent. Bake for about 45-50 minutes. Uncover and bake for another 10 minutes or until golden on top.

Note: I also add a little s&p to the cream. Sometimes I just smash the garlic cloves to infuse into the cream, them remove them along with the thyme sprig after heating. Baking time can vary a bit; just keep an eye on it.

ETA - Just thinking...Ina has a Potato-Fennel Gratin recipe. I haven't tried it but it might be worth checking. If you're not a fennel fan, you could just sub with more potatoes.

https://barefootcontessa.com/recipes/potato-fennel-gratin-updated