Toast the Bread: Preheat your oven to 400°F (200°C) or heat a grill pan over medium-high heat. Brush both sides of the sliced bread rounds lightly with olive oil. Bake or grill for 6–8 minutes until golden and thoroughly crispy. Rub the cut side of the raw garlic clove immediately across the top of each hot slice.
Whip the Ricotta: Combine the whole milk ricotta cheese, chopped chives, lemon zest, salt, and pepper in a small bowl. Stir vigorously with a spoon or small whisk until smooth, fluffy, and creamy. Set aside.
Sauté the Mushrooms: Heat the olive oil or butter in a large skillet over medium-high heat. Add the sliced mushroom medley in a single layer and cook without moving them for 3 minutes to get a deep brown, caramelized sear. Stir the mushrooms and cook for another 3 minutes until fully tender. Turn heat down slightly and add the minced garlic, chopped sun-dried tomatoes, and red pepper flakes. Sauté for 1 more minute until fragrant. Toss in the fresh parsley, season with salt and pepper to taste, and remove from heat.
Assemble the Crostini: Spread a thick, generous layer of the seasoned ricotta cheese over each garlic-rubbed toasted bread slice. Spoon the warm, savory mushroom mixture over the cheese. Garnish with extra parsley and a light sprinkle of flaky sea salt before serving immediately.