Preheat oven to 350°. Butter a 2-quart shallow baking dish.
Wash, derib and coarsely chop the kale.
Place in a pot with a little bit of water and steam until almost tender, 5 minutes or so. Check to make sure it's not burning.
Peel and cut potatoes and onions into 1/8-inch thick rounds.
If desired, saute onions until lightly browned. It gives more flavor, I think.
Drain kale and let stand until cool enough to handle. Press out excess water.
Build up alternating layers of potatoes, onions and kale (two layers each) in baking dish, beginning and ending with potatoes and dotting each onion layer with butter, minced tarragon, salt, pepper and garlic powder.
Pour half-and-half over top, cover and bake in oven until potatoes are tender and almost all liquid is absorbed, 30-45 minutes.
Place under broiler, if desired, to brown the top.