Preheat the oven to 350┬░F. Butter a 2-quart gratin dish.
Have ready:
4 medium Yukon gold potatoes or all-purpose potatoes (about 5 ounces each), peeled and cut into 1/8 -inch-thick rounds
2 small onions, cut into 1/8-inch-thick slices
Steam until almost tender, 8 to 10 minutes:
1 large bunch kale (about 1 pound per bunch), stemmed and washed
Drain and let stand until cool enough to handle. Press out the excess water and coarsely chop. In the gratin dish, build up alternating layers of potatoes, onions, and kale, beginning and ending with the potatoes. Dot each onion layer with:
1 tablespoon butter, cut into pieces
1/2 teaspoon minced fresh tarragon
1/4 teaspoon salt
1/8 teaspoon ground black pepper
Pour over the layers:
1 1/2 cups milk or half-and-half
Cover and bake until the potatoes are tender and almost all the liquid is absorbed, 30 to 45 minutes. Broil, if desired, to brown the top. Serve.
