This is a great casual lunch or supper for entertaining or a picnic since everything can be made ahead. You can substitute tortilla bread for the pita and wrap the sandwich instead of a pita pocket.
Add olive oil to a medium sauté pan over medium high heat. Saute onions until soft, about 3 to 4 minutes. Add mushrooms, thyme and tarragon and white wine. Reduce heat to low. Stirring frequently, simmer uncovered until all the moisture has evaporated, about 20 minutes. Season confit with salt and pepper to taste. Set aside to cool.
Preheat oven to 375 degrees F.
Add garlic, soy sauce, lime juice, thyme and mustard into a bowl. Stir to combine. Add the chicken and rub with soy mixture. Season the chicken with salt and pepper.
Heat oil in a medium skillet over high heat. When oil is hot, sear chicken on each side until evenly browned, about 2 minutes per side. Transfer chicken to a baking pan and finish cooking in the oven until chicken is just cooked through, about 7 to 9 minutes. Set aside and let cool to room temperature. Slice chicken diagonally into three or four pieces.
Cut pitas and serve chicken in pita pockets with slices of tomatoes and the mushroom confit.