While you can prepare the mushroom topping in advance of serving this, I do not recommend assembling the toast until just before serving. If you enjoy this flavor profile, you can easily make a larger batch of the mushroom mixture and refrigerate it for 3 to 4 days. When ready to serve, just toast the bread, spread the ricotta, and reheat the mushrooms in the microwave or on the stovetop to go on top of the toast.