This side dish recipe, inspired by Yotam Ottolenghi's recipe, celebrates the natural sweetness of the onion. I realize this may be a tough sell with some people, but the red onions are quite sweet, and when roasted they taste even more delicious and make a beautiful topping for a salad with intense flavor.
Preheat the oven to 400°F.
Cut off the tops and bottoms of the onions, slice each onion crossways into 1-inch slices and place in a roasting pan. Brush with the olive oil, sprinkle with 1/4 teaspoon salt and some pepper, and roast for 20-30 minutes, until cooked and golden brown on top.
Set aside to cool slightly and place on a serving platter.
While onions are roasting, stir parsley, walnuts, chile (if using), garlic, vinegar, oil, and sea salt together in a small bowl and spoon over the roasted onions. Sprinkle with feta or goat cheese to serve.