Melt butter in a saucepan over medium-high heat. Stir in onion, and cook until the onion has begun to turn golden brown, about 5 minutes. Meanwhile, place broccoli in a steamer over 1 inch of boiling water, and cover. Cook until tender but still firm, about 6 minutes. Drain.

When the onions have turned golden brown, remove the saucepan from the heat, and whisk in the sour cream, vinegar, sugar, poppy seeds, paprika, salt, and cayenne pepper. Place the cooked broccoli into a serving dish, and pour the sauce overtop. Sprinkle with chopped pecans to serve.