Preheat the oven to 220 ° C (top / bottom heat). Wake the potatoes and cut them into 4 or 6 wedges, depending on their size. Cut the onions into 8 wedges.
Combine oil, salt, pepper and rosemary in a bowl. Add the potatoes and onions and toss until everything is well covered with the marinade. Transfer to a baking sheet and bake for 35 minutes. Sprinkle the sunflower seeds on the baking tray in the last 5 minutes.
Wash the spinach and roughly chop it into bite-sized pieces. Mix the dressing ingredients well.
When the potatoes are ready, mix the spinach with the dressing in a large bowl, then mix in the potatoes. Serve warm.