Polenta With Mushrooms and Asparagus

itemscope itemtype="http://data-vocabulary.org/RecipeIngredient"> Cooking spray 6 ounces polenta in tube, cut into 4 (1/2-inch-thick) slices 6 asparagus[...]

2 tablespoons unsalted butter , plus extra for the baking dish 2 cups polenta or 2 cups coarsely ground yellow cornmeal 1 quart water or 1 quart chicken stock[...]

1 cup quick-cooking polenta 1 small onion , chopped 1 tablespoon olive oil 3 cups fresh mushrooms , sliced (8 oz.) 1 lb asparagus spear , trimmed and cut into[...]