For the fish: Place 2 lemon slices in the center of a large square of parchment. Lay a piece of fish on top. Sprinkle with salt and top with 1 tablespoon butter and half the thyme. Fold the parchment paper over the fish and carefully twist and seal into a packet, being careful not to move the filling inside. Repeat with the remaining fillet of salmon, butter and thyme. Brush the outside of the parchment packets with the olive oil and place on a sheet pan.
For the vegetables: Spread the asparagus and radishes on a second sheet pan. Sprinkle with salt and pepper and drizzle with the olive oil.
Place the pan with the fish on the middle rack of the oven. Place the pan with the vegetables on the top rack. Bake until the fish is cooked to an internal temperature of 120 degrees F and the vegetables are cooked to crisp-tender, 18 to 20 minutes. Squeeze the juice from the lemon half over the vegetables.
For the hollandaise: Place the egg yolks, lemon juice, salt and a splash of water in a blender. Blend on high for about 30 seconds to incorporate everything and froth up the yolks. Turn the blender to medium speed and slowly drizzle in the butter. This will emulsify the butter into the eggs and complete the hollandaise sauce. Add the mustard and blend just to combine.
Gently remove the fish from the parchment envelopes and place on a rack fitted inside a sheet pan. Gently spoon an even layer of the hollandaise over the fish. Arrange the asparagus, radishes and salmon roe around the edges of 2 plates. Sprinkle with the chives. Place a piece of fish in the center of each plate in the middle of the vegetables. Garnish the plates with watercress before serving.