Step 1Heat olive oil in medium skillet over medium heat. When hot, add shallots and saute until tender, about 1 minute. Add chard and cook, stirring, until chard begins to wilt, about 1 minute. Cover, reduce heat to low and let chard steam gently about 10 minutes. Stir in lemon juice and salt to taste.
Step 2Combine mayonnaise and chili sauce. Spread dressing on 1 side of each slice bread. Place half of corned beef on each of 2 slices bread. Add 1/4 of cheese and half of chard to each. Top each with remaining cheese and remaining slices of bread. Brush top slices of bread with half of melted butter. Place sandwiches buttered side down on preheated grill pan over medium heat and grill until browned, 4 to 5 minutes. Brush top of bread with remaining melted butter, turn and grill other side, about 4 more minutes.