In a medium sauté pan, melt butter over medium heat. Add shallot, parsley, rosemary, and garlic; cook until fragrant, approximately 3 minutes. Add mushrooms, salt, and pepper; cook for 6 minutes, stirring occasionally. Remove from heat, and add vinegar, stirring well. Transfer mixture to a heatproof bowl. Let cool to room temperature.
In the work bowl of a food processor, pulse together mushroom mixture and walnuts until a purée forms. Store in an airtight container in the refrigerator overnight, 6 to 8 hours, before using.
Recipe by TeaTime Magazine at https://teatimemagazine.com/mushroom-steak-canapes/