Cut any smaller mushrooms into halves or quarters and cut any larger ones into bite-sized pieces.
Heat the oil and butter in the insert pan of a slow cooker or a large frying pan over medium heat. Cook the shallots, garlic and paprika for 2 minutes. Add the mushrooms and cook for 3–4 minutes until starting to soften. Add the flour and cook for 2 minutes.
Return the insert pan, if using, to the slow cooker, or transfer the mixture to the slow cooker with the vegetable stock, tomato paste and crème fraîche. Season with sea salt and freshly ground black pepper, stir well to combine, then cook on low for 3 hours.
Spoon the mushroom stroganoff over your choice of pasta, rice or mashed potato and scatter over the parsley.