Mushroom Matzo Kugel

Name: Mushroom Matzo Kugel
Ingredients: ½ small onion; 2 sprigs parsley; 1 dried porcini mushroom; ¼ cup mixed dried mushrooms; 1 tsp. black peppercorns; ½ tsp. Diamond Crystal or ¼ tsp. Morton kosher salt; 8 Tbsp. (1 stick) unsalted butter, divided, plus more for pan; 1 11-oz. box matzo (about 13 boards); 5 large eggs; 1 cup crème fraîche; 1 cup heavy cream; Kosher salt; 1 large bunch Swiss chard; 10 oz. shiitake mushrooms, stems removed, sliced ¼ thick; 8 oz. maitake mushrooms, cut into bite-size pieces; 1 medium onion, finely chopped