1. Whisk together vinegar, oil, garlic powder, salt and pepper in a small bowl.
2. Put vinegar mixture and clean mushrooms in a large, resealable plastic bag and shake to coat. Let marinate for at least 30 minutes at room temperature or up to four hours in the refrigerator, turning occasionally.
3. Preheat grill to medium-high heat. Gently shake off excess marinade and grill mushrooms for 4-5 minutes per side. Cool briefly, then slice mushrooms.
4. Preheat broiler. Place ciabatta slices on sheet pan. Evenly divide mushroom slices on ciabatta.
5. Top each piece of ciabatta with two slices of Kerrygold Swiss Cheese. Broil until cheese melts.
6. Top each toast with arugula and pickled red onions. Drizzle with grainy mustard and sprinkle with flaky sea salt.
Pickled Red Onions:
1. Place vinegar, sugar and salt in a small saucepan and cook over medium-high heat for 3 minutes or until sugar dissolves, stirring occasionally.
2. Place peppercorns and onions in a jar or other lidded container. Pour hot vinegar mixture over onions and cool to room temperature. Cover and refrigerate until ready to use.
Note: onions will keep for up to 3 weeks in the refrigerator.
Recipe by Garnish with Lemon