Light a grill. Soak the cedar planks in water for 30 minutes. When the coals are covered with a light gray ash, push them to opposite sides of the grill and set a disposable drip pan in the center.
Meanwhile, in a small bowl, blend the whole-grain mustard with the mayonnaise, dry mustard, soy sauce, honey and lemon zest; season with salt and pepper. Season the salmon fillets with salt and pepper.
Drain the planks and pat dry. Set the salmon fillets on the planks, skinned side down. Spread the mustard glaze over the top and sides of each fillet. Arrange the planks on the hot grate above the drip pan and away from the coals. Cover and cook for 15 minutes or until the salmon is browned and just cooked through. Using a long metal spatula, transfer the salmon to plates and serve.
The mustard glaze can be refrigerated overnight.
Oak-barrel aging can impart a toasty quality to Chardonnay that matches ideally with the cedary smokiness of these salmon fillets.