Preheat oven to 350 degrees F. Grease a 2-quart gratin dish or 2-quart rectangular baking dish; set aside.
Halve pineapple lengthwise; thinly slice one of the halves. (Reserve remaining half for another use.) Heat a small saucepan over medium heat. Add cumin; cook for 1 to 2 minutes or until toasted. Add cream, poblano, garlic, salt, and pepper. Heat through; set aside.
Layer half of the potatoes, pineapple, and green onions in prepared dish. Sprinkle with half of the cheese. Repeat layers. Pour cream mixture over layers. Bake, covered, for 25 minutes.
In a small bowl combine panko and oil. Sprinkle over the gratin. Bake, uncovered, 55 minutes more or until potatoes are tender and top is golden. Let stand 10 minutes before serving.