Boil carrots for 6 minutes;reserve 1/4 liquid.
Drain carrots; arrange them inside a 10x6 dish. In a small bowl, wisk onioni, horseradish, mayo, and pepper. Add extra liquid. Pour the enire misture over carrots.
Stir bread crumbs into butter, sprinkle over carrots.
Bake in a 375 degree oven for 15 minutes/