Cut six millimeters off the top off the head of garlic to expose the cloves and discard. Place garlic in the center of a foil sheet. Sprinkle salt and pepper over the cloves and drizzle with one tablespoon (15 milliliters) olive oil. Fold foil to seal securely. Place in a small ovenproof dish and roast for 30 minutes. Set aside and let cool.
In a small skillet, heat remaining oil over medium heat. Add sage leaves and fry until crisp, about one minute. Transfer to paper towels to drain. Crush the leaves in a small dish and set aside.
In a four litre pot, place cassavas and cover with cold salted water over high heat. Bring to a boil. Reduce heat to medium and cook cassavas, about 30 minutes or until tender when pierced with a paring knife. Drain. Transfer to a large bowl and mash. Mash garlic and fold into mashed cassavas.
In a small one litre saucepan, add cream and butter over medium heat and cook, stirring, until cream thickens slightly or forms tiny bubbles. Add cheese and season to taste with salt and pepper. Remove pan from heat. Pour over cassavas and mix thoroughly. Serve at once.