Preheat the oven to 375°F. Brush the slices of bread with the olive oil, sprinkle with salt and bake for about 5 minutes until a touch golden around the edges, but not firm – they will firm up a bit as it cools. Transfer the bruschetta to a cooling rack or plate.
Melt the butter in a large skillet over medium high heat. Add the mushrooms, season with salt and pepper, and sauté for about 8 minutes until any liquid released has evaporated, and the mushrooms are nicely browned. Add the garlic and saute for one more minute until you can smell the garlic. Remove from the heat.
Spread some of the Herbed Mayonnaise on the toasts and heap them with the mushrooms (ideally while the mushrooms are warm, but not necessarily). Serve quickly so the bread doesn’t get soggy.