Preheat oven to 350°F.
Heat a large skillet over medium-high heat. Add 1 tablespoons olive oil and allow to heat through. Throw in the sausage and crumble with a wooden spoon. Cook until browned and crispy, about 10 minutes. Transfer to a bowl or plate and set aside. Add the chopped onions, garlic and mushrooms to the skillet. Sauté until soft and just beginning to brown. Season with salt, pepper, fresh thyme and sage. Cook for another 1 to 2 minutes. Remove from heat and set aside.
Bring a large pot of water to a boil. Lightly season it and throw in the green beans. Cook until tender and bring green. Drain and immediately transfer to a bowl filled with cold water and ice. Drain once more and allow to air dry.
To make the pesto, throw into all of the ingredients (except the olive oil) into a food processor or blender. Pulse until chopped. While the machine is running, stream in the olive oil until smooth and desired consistency is reached. It should look like a thinner ricotta, but not too thin. Taste for seasoning and adjust accordingly. Transfer to a bowl. (Can be made a few days in advance and stored in the fridge).
Cut the bread into thick slices on an angle and arrange on a baking sheet. Drizzle the remaining 2 tablespoons olive oil over the bread and bake in the oven for about 5 to 10 minutes, flipping halfway. It should be golden brown and crusty.
If using fried sage leaves as garnish, heat a skillet over medium heat. Add a bit of oil, and allow to heat through. Add the sage leaves, whole, and fry for a few seconds until crispy. Drain on paper towels and use as garnish.
To assemble the crostini, spread each slice with ricotta, top with pea pesto, sausage and mushroom filling, parmesan cheese and fried sage, if using. Serve at room temperature or hot. Can be made in advance, kept separately in the fridge and then arranged right before serving. Enjoy!