CUT squash into quarters. Scoop out and discard seeds. Roast pieces in a large baking dish covered with foil, until fork-tender, about 1 hour. Remove from oven.
WHEN cool enough to handle, scoop flesh from skins. Whirl in a food processor until smooth. (Refrigerates for up to 3 days.)
COMBINE 2 cups butternut squash purée with maple syrup, butter, cinnamon, ginger and salt in a large non-stick frying pan. Boil, then reduce heat to medium. Cook, stirring frequently, until mixture thickens and turns dark brown, 18 to 20 min. Spread on toast. Sprinkle with flaked sea salt.