Saute the onions and garlic in butter until golden. Add the potatoes, parsnips, carrots, turmeric, curry powder, and stock or water and simmer until the vegetables are tender, about 20 minutes. Reserve 1 cup of the peas for garnish and add the rest to the pot. Cook frozen peas for 10 minutes, fresh ones for 5 minutes. In a blender or food processor, puree the hot soup with the half-and-half until very smooth. Add salt and pepper to taste. Cook the reserved peas until just tender to garnish individual servings.