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1
Chop the roasted bell pepper coarsely.
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2
In a blender or food processor, combine the roasted pepper, shallot, walnuts, 1 tablespoon of the vinegar, the mustard, 1/4 teaspoon of the salt and 1/4 teaspoon of the pepper.
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3
Pulse a few times to puree and set aside.
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4
In a large, nonstick frying pan, heat 1 1/2 teaspoons of the olive oil over medium-high heat.
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5
Season the chicken breasts with the remaining 1/8 teaspoon salt and 1/8 teaspoon pepper.
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6
Add the chicken to the pan and cook, turning once, until lightly browned on both sides and no longer pink on the inside (170 F if using a food thermometer), 3 to 4 minutes on each side.
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7
Transfer to a cutting board and let cool.
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8
Thinly slice the chicken lengthwise into strips.
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9
In the same frying pan, heat the remaining 1 1/2 tablespoons olive oil over medium-high heat.
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10
Add the mushrooms and saute, turning frequently, until well browned, 7 to 9 minutes.
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11
Remove from the heat and drizzle with the remaining 1 1/2 teaspoons balsamic vinegar.
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12
Using a serrated knife, split the baguette sections in half lengthwise.
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13
Spread about 1 tablespoon red pepper mixture on both halves of each baguette section.
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14
To assemble a sandwich, place 1 mushroom half on the bottom half of a baguette section.
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15
Top with 1/4 of the chicken, a lettuce leaf, and 1 or 2 tomato slices.
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16
Close the sandwich with the top half of the baguette section, and slice in half.
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17
Repeat to make 3 more sandwiches, and then serve.
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18
If making ahead, omit the red pepper mixture, wrap the sandwiches in plastic wrap and refrigerate.
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19
Spread the red pepper mixture on the baguettes just before serving.
