From the blog For Love of the Table
1 T. olive oil, more as needed
1 small (or half of a large) bulb of fennel (weight when trimmed of stalks should be about 6 oz.), halved, cored and sliced very thinly (use a mandolin) crosswise—you should have about 1 cup of sliced fennel
1 small (or half of a medium) onion (4 oz.), thinly sliced
2 cloves garlic, thinly sliced cross-wise
Salt & pepper to taste
2 small or 1 large tomato (1/2 lb.)
1 1/2 lb. russet or Yukon gold potatoes
2 to 3 t. picked thyme, coarsely chopped
1/2 c. Heavy cream
1/2 to 3/4 c. chicken (or vegetable) stock
1/3 c. grated Parmesan (1 oz.)
Warm the olive oil in a medium sauté pan set over medium heat. Add the fennel, onion and garlic along with a pinch of salt and toss to coat in the oil. When the vegetables begin to sizzle, reduce the heat to low, cover and cook until just tender—about 15 to 20 minutes. Set aside.
While the onion and fennel cooks, peel the tomatoes (by blanching and shocking or using the method outlined in this post). Place a sieve over a bowl, halve the tomatoes crosswise and remove the seeds while holding the tomato halves over the bowl. Thinly slice the tomatoes halves and reserve separately with their juices.
To build the gratin, lightly oil a shallow 1 1/2 quart baking dish. Peel the potatoes and using a mandolin, slice the potatoes crosswise very thinly (1/16-inch). Shingle half of the potatoes into the gratin in overlapping rows. Season with salt, pepper and a third of the thyme. Spread the fennel-onion mixture over the potatoes in an even layer and season in a similar manner. Next, layer in the tomatoes, spreading evenly. Pour the reserved tomato juices over the tomatoes. Season with salt, pepper and the remaining thyme. Layer in the remaining potatoes and season with salt and pepper. Pour the cream over all, jiggling the dish slightly to make sure the cream penetrates all of the layers of vegetables. Pour in enough chicken stock so that the vegetables in the middle are just barely covered with liquid when pressed with a spatula or your hands.
Cover the gratin tightly with foil and place on a baking sheet. Place in the center of a preheated 375° oven and bake until the cream and stock are bubbling around the edges—about 30 to 40 minutes. Remove the foil, scatter the Parmesan evenly over the gratin and continue to bake until the cream is reduced and bubbling thickly, the top is a beautiful golden brown and the potatoes are tender to the tip of a knife—another 30 to 40 minutes. Cool briefly before serving. Serves 4 to 6.
Notes: To make the gratin without heavy cream simply omit it and add stock to the appropriate level in the dish—you will need 1 to 1 1/4 cups of stock. Drizzle the gratin liberally with olive oil before covering with foil. I have never tried it, but I think this all-stock version would be delicious with saffron. To make, steep a pinch of saffron 3/4 cup of the hot stock before pouring it over the gratin. Again, add as much more plain stock as is necessary to properly moisten the gratin.
http://www.forloveofthetable.com/2013/10/potato-gratin-with-fennel-tomatoes.html