Complex sentence: They took every seat in Atlantic Canada, swept Toronto, won all but one seat in Ottawa and most notably, won 40 of Quebec's 78 seats, mostly on the strength of taking all but five seats in Montreal.
Split into simpler sentences: They took every seat in Atlantic Canada and Toronto, and won all but one seat in Ottawa. Most notably, they won 40 of Quebec's 78 seats, mostly on the strength of taking all but five seats in Montreal.