Lightly brush both sides of the bread with 2 tablespoons olive and place on a baking sheet. Bake for 8 minutes on one side, flip the bread and bake for another 8-10 minutes, until crisp and lightly golden. Set aside.
Trim the root and dark green leaves from the leek. Slice in half, vertically, then slice into 1/4" half moons. Transfer the leeks to a bowl of water and use your hands to swish the leeks in the water to remove any dirt or grit. The dirt will sink to the bottom of the bowl. Lift the leeks out of the water and transfer to a bowl lined with paper towels to dry.
Meanwhile, slice the mushrooms into 1/4" slices and prep the herbs and garlic.
Heat the remaining two tablespoons olive oil over medium heat in a heavy saucepan with a lid. Add the mushrooms and leeks and sprinkle with the kosher salt. Stir and place the lid on the pan to sweat the vegetables. Check the vegetables every 2 minutes, stirring until the mushrooms and leeks have softened. 5-8 minutes. Add the thyme, rosemary and garlic and cook for 2-3 minutes until softened and fragrant. Place the lid on the pan and turn off the heat and let the vegetables rest.
In a small bowl mix the goat cheese, cream and black pepper. Stir until smooth, creamy and spreadable. Spread a small spoonful of goat cheese onto the crostini. Top with mushroom mixture. Can be made 2-3 hours in advance to this point and refrigerated until ready to serve.
When you're ready to serve, heat the crostini for 5 minutes at 350 degrees to warm them through. Garnish with chives and serve,