With your steep slope and with new loose top soil, it sounds as if you would benefit from using some sort of erosion control blanket. The erosion-control fabric works by slowing the runoff water and allowing sediments to fall out rather than be washed away. Seeds are sown under the erosion-control material and grow up through the matting when they germinate. You can insert plants into the soil by cutting through the matting. The roots of the plants that are growing through the erosion-control material anchor the soil to stop the erosion. If you use erosion-control blankets made of biodegrable material, they will eventually disappear leaving the plants to control the problem. Most nurseries carry these erosion control blankets.
Let's start with grasses since the extensive fibrous roots of grasses are very effective in holding soil in place. Here are some native New York grasses that should do well:
You can add other plants to the grasses, but since I don't know how much sunlight you have on your slope and how much moisture there is in the soil, I can only make general suggestions. You should check the GROWING CONDITIONS for each recommended plant to be sure that they match the conditions at your site. Here are some plants that grow in or adjacent to Onondaga County, New York that would be suitable to add to the slope.
You can find other possibilities on our New York Recommended page.
Here are photos from our Image Gallery: