Bring a large pot of water to boil and add 1 tbsp. salt. Fill a large bowl with ice water and set aside. Select the 12 largest chard leaves (save remaining chard for another use) and cook in boiling water 1 minute. With tongs or a slotted spoon, transfer chard to ice water.
Drain chard and spread leaves out on clean, dry kitchen towels.
Arrange one large leaf on your workspace horizontally and arrange a second leaf on top of and perpendicular to the first. Sprinkle some of the parsley and thyme down center of leaves, then top with 1 1/2 tsp. butter. Generously season halibut with salt and pepper and set 1 fillet on butter and herbs. Arrange 4 lemon slices over top of fillet. Fold chard leaves around fish to enclose. Place packet seam side down in a steamer basket large enough to accommodate all six packets (or use a two-tiered bamboo steamer). Repeat with remaining 5 fillets.
Fill a large, high-sided frying pan or a wok with enough water to come 3/4 in. up sides and bring water to a boil. Add steamer basket and cover. Steam fish until just cooked through, 6 to 8 minutes. Very carefully cut into one packet to check fish for doneness.
While fish is steaming, make beurre blanc: Put shallot and lemon juice in a small nonreactive saucepan over medium heat. Cook until liquid is reduced to 1 tbsp. Remove from heat and whisk in remaining 5 tbsp. butter, 1 tbsp. at a time, until emulsified. Season with salt and pepper.
Serve fish packets immediately, topped with beurre blanc.
Note: Nutritional analysis is per serving.