Preheat oven to 375°F.

Place carrots in saucepan with just enough water to cover. Summer until just tender. Drain and reserve 1/4 C cooking liquid.

Mix reserved liquid with horseradish, salt & pepper, onion, and mayo

Arrange carrots on bottom of a buttered 9" pie plate. Pour horseradish mixture over carrots. Mix breadcrumbs with butter and sprinkle over top.

Bake for 20 minutes, or until crumbs are browned.