Make the cracker dough: Combine the flour, parmesan, sesame seeds, nigella seeds, thyme, black pepper, and salt in a bowl. Add the olive oil and mix until slightly crumbly. Add the cold water and bring together into a soft dough. Knead briefly until smooth, then cover and rest for 20–30 minutes.
Make the mushroom pate: Finely chop the mushrooms (or pulse briefly in a processor). Cook in a wide pan until all moisture has evaporated. Add olive oil and 1 tbsp of the butter and cook until deeply browned and concentrated.
Add the shallots and garlic and cook until soft. Stir in the thyme and the brandy or port, and cook until reduced. Add the remaining 1 tbsp butter, the soy sauce, and the balsamic vinegar, and cook briefly until the butter has melted. Remove from the heat.
Allow the mixture to cool slightly, then transfer to a blender. Blend briefly, keeping some texture, drizzling in the cream as you blend to help bring the mixture together into a smooth, spreadable consistency. Fold in the toasted hazelnuts and chopped chives. Season with salt and black pepper to taste, then chill for 30–60 minutes.
Bake the crackers: Preheat the oven to 170°C fan (190°C conventional). Roll the dough very thin (1–2mm) between sheets of baking paper. Leave whole to break apart later, or cut into shapes. Bake for 12–15 minutes until golden and crisp. Cool completely on a rack.
Serve: Spoon the pate into a bowl. Using a mandoline, shave paper-thin slices of the reserved raw mushroom and use as a garnish. Serve with the crackers.