Rub steaks with about half the steak seasoning; place on a plate and refrigerate.
Light an outdoor charcoal grill.
Heat butter in a skillet over medium-low heat; cook and stir mushrooms, shallots, and garlic until mushrooms are softened, 5 to 10 minutes. Remove from heat.
Move heated charcoals using tongs to the outer edges of the grill. Fill a 9x5-inch loaf pan halfway with water; stir in the remaining 1/4 cup steak seasoning. Carefully place pan in the space cleared out in the center of the grill among the charcoals.
Cook steaks on the grill until they are beginning to firm, and are hot and slightly pink in the center, 8 to 9 minutes. Flip and cook other side for 5 to 7 minutes. An instant-read thermometer inserted into the center should read 140 degrees F (60 degrees C). Drizzle mushroom sauce over cooked steaks.