Preheat oven to 400°; slice half the potatoes in half lengthwise, mix with 1 tbsp oil, salt/pep + fave spices (we like garlic powder), then spread on a sheet pan cut side down; roast 25-30 min until tender

Meanwhile, cut onion in half from root to stem; peel + thinly slice ½ of onion + set aside

Divide potatoes to plates; separate slices of tuna + scatter amongst potatoes

Wash + dry parsley, remove stems + roughly chop; sprinkle onion slices + parsley over each dish + dollop with sour cream