- Heat oven to 160 degrees C (Gas Mark 3).
- Butter a 9 1/2 x 4 1/2 x 2 1/2 inch loaf pan, line bottom with parchment paper, and butter again; set aside.
- Place 2/3rds dried porcini in a small bowl; add boiling water to cover.
- Let soak 20 minutes; drain, finely chop, and set aside.
- Toast walnuts and cashews on a baking sheet until lightly browned and fragrant, about 10 minutes. Let cool.
- Finely chop with a sharp knife, or in the bowl of a food processor, set aside.
- Increase oven heat to 170 degrees C (Gas Mark 4).
- Melt 2/3rds of butter in a sauté pan over medium heat.
- Add onion and garlic; cook until translucent, about 3 minutes.
- Add shiitake mushrooms and the reserved porcini; cook, stirring constantly, until all the mushrooms are golden, about 5 minutes.
- Transfer to a large bowl. Stir in rice, the toasted nuts, parsley, marjoram, thyme, sage, eggs, Gruyère, cottage cheese, salt, and pepper.
- Pour mixture into prepared pan, and bake until golden brown and firm to the touch, about 1 hour.
- Let cool in pan 20 minutes, then invert loaf, and remove from pan.
Serve hot or warm, garnished with sage leaves.