Cut 12 oz. fingerling potatoes in half lengthwise; place in a microwavable bowl with 1/4 cup water. Cover with plastic wrap, and pierce with a fork to vent. Microwave until tender, 6 to 7 minutes; drain. Combine potatoes, thinly sliced red onion, fresh flat-leaf parsley leaves, olive oil, chopped fresh rosemary, black pepper, and kosher salt in a bowl. Flake smoked trout into large pieces; gently fold into potato mixture.