Heat oil in a large saucepan over medium heat, add onion and garlic, and sauté until tender (10 minutes). Add pumpkin and stock, bring to the simmer, season to taste, reduce heat to low, then cover and cook until pumpkin is very tender (40-45 minutes). Add half the fontina, stir to melt, season to taste and keep warm.
Preheat grill to high. Combine parmesan, mascarpone, sage, rosemary, egg yolk and remaining fontina in a bowl, season to taste.
Divide soup among ovenproof bowls. Place bread on a tray and grill until golden (1 minute each side). Spread with cheese mixture, then position on top of soup and place bowls under grill until cheese is golden and melted (1-2 minutes). Serve hot.