Step 1

1 Whisk together the mustard, vinegar and oil in a medium bowl until emulsified

Step 2

2 Add the mushroom slices and toss to coat evenly

Step 3

3 Let them marinate for 15 to 30 minutes while you prepare the grill and char the poblano

Step 4

4 Prepare the grill for direct heat

Step 5

5 If using a gas grill, preheat to high (650 degrees)

Step 6

6 If using a charcoal grill, light the charcoal or wood briquettes; when the briquettes are almost completely covered in white ash, mound them in the center of the grill

Step 7

7 For a very hot fire, you should be able to hold your hand about 6 inches above the coals for 1 or 2 seconds

Step 8

8 Have ready a spray water bottle for taming any flames

Step 9

9 Lightly coat the grill rack with oil and place it on the grill

Step 10

10 Grill the poblano pepper, turning it every few minutes, until it is charred on all sides, about 10 minutes total

Step 11

11 Transfer to a small bowl and cover tightly with plastic wrap

Step 12

12 If using a gas grill, reduce the heat to medium-high (450 degrees)

Step 13

13 If using a charcoal grill, carefully remove the grill rack and spread the coals into an even layer, carefully removing some if necessary to achieve a medium-hot fire; you should be able to hold your hand about 6 inches above the coals for 4 or 5 seconds

Step 14

14 Remove the mushroom slices from the marinade and grill them until deeply browned, about 5 minutes per side

Step 15

15 Transfer to a cutting board to cool, then chop them into bite-size pieces

Step 16

16 When the charred poblano is cool enough to handle, use your fingers to slip off the blackened skin (resist the urge to rinse the pepper, which would remove too much flavor), then discard the stem, tear open the pepper, and scoop out and discard the seeds

Step 17

17 Cut the pepper into bite-size strips

Step 18

18 If you have any marinade left, brush it on one side of each slice of bread

Step 19

19 With the brushed side facing down, layer one slice of the bread with half of the cheese, followed by the mushroom pieces, poblano, pickle slices and the rest of the cheese

Step 20

20 Top with the other bread slice (brushed side facing up), and press the sandwich with your hand to flatten it

Step 21

21 Grill the sandwich until the bread is golden brown and the cheese melts, 2 to 4 minutes per side

Step 22

22 Transfer to a plate, cut in half diagonally, and eat