Melt the butter in a large pot over medium heat. Add the thyme, garlic and sweet potatoes. Cook the potatoes until they start to brown, about 6 minutes. Add the dill, parsley, scallions, Spanish onions, chives and red onions and cook until the onions turn translucent, about 4 minutes. Add the pesto, black pepper, smoked paprika and cayenne, stirring to combine. Stir in the milk and bring to a simmer, cooking until the potatoes are tender, 8 to 10 minutes. Turn off the heat. Fold in the cream and flaked whitefish. Season with more black pepper if necessary. Garnish with chives and serve it while it's hot!