Don't know what you consider high-end, but we had incredible sushi in the following.:

Otaru: Takara-Sushi

Sapporo: Kin Sushi (Some of the most unusual we have had). Fourth generation owner served us at the sushi bar. Witnessed tourists turned away who did not have reservations.

Note: Both of the above are very local places in which little English is spoken. Reservations are really necessary.

Now if you want a Michelin Kaiseki restaurant in Sapporo: Hanakoji Sawada. Best Michelin bargain. Not sure when you are going, but during the Snow Festival, the only way we got a reservation is that our guide made reservations in advance and personally vouched for us and guaranteed our presence.

Very nice and local tempura restaurant in Sapporo: Tempura Takeuchi. Best tempura we have ever had.