Heat the olive oil in a large sauté pan over medium-high heat. Add the shallot and cook until translucent, about 1 minute. Add the mushrooms and cook, stirring, until they give off their liquid and are golden brown, about 6 minutes. Add the marjoram, garlic and some salt and pepper. Stir and cook for 1 minute more.
Meanwhile, add the ricotta, Parmesan, lemon zest and juice and salt and pepper to taste to a food processor and process until very smooth.
Spread the whipped ricotta on the toasted bread. Toss the arugula with salt and pepper. Arrange the mushrooms on the ricotta mixture, then top with the arugula.