Preheat the oven to 400ºF and line a large baking sheet with parchment paper. Lay the sliced potatoes flat on the pan, season generously with salt and pepper. When the oven is preheated, bake the potatoes for 35-40 minutes until the potatoes are golden brown and can be pierced with a fork.
Meanwhile make the white bean arugula cream. Sauté the onions for about 10 minutes in 2-3 tablespoons of water until they are translucent. Add the navy beans, cooked onions, nutritional yeast, white balsamic, salt and red pepper flakes to a food processor fitted with the S blade. Process until smooth. Add reserved bean liquid, 1 tablespoon at a time, to reach a creamy consistency. Scrape down the sides as needed. Next add the arugula and pulse 7-10 times until incorporated. Transfer the cream to a glass dish and set aside.
Lastly, make the mushrooms. Add the sliced mushrooms to medium sized pan and cook on medium heat with 2-3 tablespoons of water and a generous pinch of salt. Once the mushrooms start releasing their liquid and shrink in size, about 4-6 minutes, add the garlic and thyme and continue to cook, stirring frequently, for another 5 minutes. Add the white balsamic vinegar and cook for another 2 minutes until all the liquid is evaporated. Remove from heat.
To assemble, carefully spread 1-2 tablespoons of the white bean arugula cream onto the hot potato crostini. Add 1 tablespoon of sautéed mushrooms and garnish with chopped arugula and fresh cracked salt and pepper! Add a dash of hot sauce if you like it spicy! Repeat with all potato crostini and enjoy!