Heat oven to 350 degrees. Spray a 2-1/2 quart baking dish with non-stick cooking spray.
In a large skillet saute onions in olive oil over medium heat until onions are tender and a light golden color. Set aside to cool.
Place potatoes and salt into a large saucepan and add water to cover. Bring to boil, then reduce to simmer until potatoes are tender. Drain. When potatoes are easy to handle, peel, then cut across into 1/4-inch slices. Arrange slices in prepared dish in over-lapping layers.
In a large bowl mix the ricotta cheese, sour cream, Romano cheese and the reserved onions. Spoon this mixture over the potatoes in an even layer. Top with the cheddar cheese. Bake in a 350 degrees oven 35-45 minutes, or until bubbly. Makes 8 servings.